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Short Explanation of the
Geological Structure of
Pennsylvania
by J.P. Lesley

 

Pennsylvania is 150 miles in width between two parallels of latitude, and 290 miles long, measuring from the meridian of the Ohio State line to either one of its two projecting points on the Delaware river, Port Jarvis at the northern angle of New Jersey , or the bend below Trenton . A slight irregularity in its northern line gives it a boundary of 50 miles radius struck form the courthouse of New Castle in Delaware takes off a small piece of its south-east corner. Its eastern line follows the course of the Delaware river .

The Allegheny Mountain divides it into two nearly equal parts, popularly know as eastern and western Pennsylvania, entirely different in geological character and in relief of surface; western Pennsylvania being one unbroken bituminous coal field, while eastern Pennsylvania is partly a labyrinth of parallel and interlocked mountains and valleys of Devonian and Silurian age, and partly an open country of still older Cambrian or Primordial, Azoic, or Fundamental strata, across which runs a broad continuous belt of Triassic or Mesozoic brown stone and trap. Cretaceous rocks underlie a narrow strip along the Delaware River below Trenton; * 1 and a mantel of glacial drift covers the surface of the whole northern part of the State as far south as a line drawn nearly straight from Belvedere on the Delaware river to Olean on the New York State line, and another line drawn nearly through Franklin and Beaver to the Ohio State line north of the Ohio river. * 2

No high mountains exist at present in Pennsylvania; its highest land may be quoted at 2500 feet above tide; and most of its ridges at 1500 to 2000 feet; although a few of the small rounded knobs along the crest of the Allegheny mountain reach 2700 and 2800. The high plateau of the Catskill in southern New York , with numerous summits exceeding 4000 feet, slopes south-westward to the Delaware river, and becomes in Pennsylvania the Pocono plateau only 2000 feet high. In like manner the great Blue Ridge range, which in North Caroline has peaks of over 7000 feet, declines through Virginia and Maryland into the South Mountain range of Pennsylvania , scarcely 2000 feet high, and ends in a point west of Harrisburg . At Reading on the Schuylkill it rises again above the surface, with parallel ridges 1000 feet high, and passes on through New Jersey to become the Highlands of the Hudson river, with summits about 3000 feet above tide.

It is noticeable, however, that at least one fourth of the State, comprising the northernmost two tiers of counties from end to end, and the tier of counties back of the Allegheny mountain, has a general surface averaging 2000 feet above the sea, until recently covered with a continuous forest, and subject to a rigorous winter climate. On this table-land the rainfall separates itself into three systems of drainage, and flows northward through the Genesee into Lake Ontario; south-eastward through the Susquehanna river into Chesapeake bay; and south-westward through the Allegheny, Ohio, and Mississippi rivers into the Gulf of Mexico.

In spite of the present nearness of the surface level of the State to the level of the sea such was not always the case. Our mountains were once ten times higher than they are now; and their gradual erosion to their present height by the frosts and rains of past ages, beginning long before the advent of the races in living beings which now inhabit the planet, makes the most interesting chapter in our geological history.

This weathering down of the surface of the State is shown by the coloring of the skeleton State map, which precedes the series of county maps. The black area in the south-west corner of this map marks those portions of Greene, Washington, Fayette, Westmoreland, Allegheny, Indiana, and Somerset counties which still preserve the Pittsburgh coal bed, and its covering of higher rocks. In several places it passes underneath the Monongahela and Youghiogheny rivers; but elsewhere these rivers have cut through it, and left its edges to be mined upon the steep side slopes of their valleys. In like manner this coal bed has been carried away from the divides, and left in long and irregular strips, and a multitude of isolated patches on the hill tops. One small patch of it is preserved in the center of the Ligonier valley in eastern Westmoreland; and several small patches of it still remain in the Salisbury basin in Somerset county.

These relics of the Pittsburgh coal bed show that is once spread over south-western Pennsylvania from the Ohio river to the Allegheny mountain. But it must have spread still farther eastward; for a large area of it is mined between Cumberland and Piedmont in Maryland; and a very small patch of it has been left on the highest summit of the Broad Top mountain in Huntington county. * 3 Finally, there are good reasons for identifying one or two of the anthracite beds with the Pittsburgh bituminous coal bed, and therefore for regarding it as a growth of swamp vegetation co-extensive with the State. Its original extent northward towards Lake Erie and New York State cannot be now learned, for it has been completely swept away from the whole Allegheny valley region, and from the upper Susquehanna river country.

The white portion of the map surrounding the Pittsburgh coal bed area

-embracing the Beaver, Lawrence, Mercer, and Crawford counties on the Ohio line; Butler, Armstrong, Clarion, Venango, Forest, Warren, and McKean, on the Allegheny river; Cambria, Indiana, Jefferson, and Elk on its eastern branches; Clearfield, Cameron, and Clinton on the Susquehanna river west of the Allegheny mountain- shows the country which is now denuded of the Pittsburgh coal bed and the beds above it, but still retain the Middle Coal Measures, the coal beds of Freeport, Kittanning, Clarion, Brookville, Mercer, and Sharon, with the great Conglomerate No. XII and the Lower Carboniferous (Pocono Sandstone No. X) down to the top of the upper Devonian, Catskill formation No. IX, which is colored red.

On the county maps it may be seen how the different coal beds, each one in its order, have been eroded away, precisely as in the case of the Pittsburgh bed, so that they now occupy only a part of their original areas. Each one in turn disappears from the sold, going northward, until at length, in McKean, Potter, Lycoming, Bradford, Tioga, Sullivan, and Wyoming, only isolated patches of the lowest workable coal beds maintain a precarious existence on the highest land, or parallel ranges of highest land, and, if the world lasts long enough, will be all slowly washed away by the rain and into the sea; their protection thus far being due to the fact that they are overlaid and underlaid by massive sand and pebble rocks which offer a stubborn resistance to erosion, and yield very slowly to the undermining action of the elements operating on the soft deposits beneath them.

Where these carboniferous sandrocks (No. XII and X) have been preserved in geological basins, run the mountain ranges, with patches of coal on their tops; where they were arched, they have been destroyed, and valleys of older, lower, softer, Devonian formations, Catskill (IX) and Chemung (VIII), have taken the place of former much higher mountains. Along the northern border of the map may be seen the extent of the Catskill red rocks, inclosing the preserved patches of carboniferous sandrocks and coal; and the erosion of the Catskill is also shown by a set of white streaks entering from the State of New York, representing valleys of Chemung exposed by the removal of the overlying Catskill. The gradual rise of the whole country north-eastward towards the Hudson river, and the increasing thickness of the Pocono and Catskill formations in that direction, has kept the underlying Chemung rocks safely covered, and this explains the unbroken spread of the red color over Wayne and the greater part of Susquehanna, Lackawanna, Monroe, and Pike counties in the north-east corner of the State.

In a deep trough in the otherwise nearly horizontal outspread of Catskill has been preserved the Coal Measures of Carbondale, Scranton, and Wilkesbarre, across Luzerne county; and so deep is this trough that it has retained not only the lower and middle, but the upper coal beds, above the Pittsburgh bed, and even a remnant of still higher rocks (containing Permian fossils) like those in Greene county, at the extreme opposite corner of the State.

In like manner the large white area on the map embracing southern Luzerne, northern Monroe and Carbon, southern Columbia and Northumberland, and north-eastern Dauphin, and nearly the whole of Schuylkill county, represents the lower carboniferous formations X and XI, lying nearly flat east of the Lehigh river, but west of that river folded into a large number of parallel arches, and an equal number of troughs, or elongated basins, in which have been preserved the Coal Measures of Hazelton and Drifton, Mahanoy and Shamokin, Tamaqua, Pottsville, Minersville, Donaldson and Wiconisco, & c. in some places to a depth of 3000 feet. These are the anthracite coal beds of eastern Pennsylvania , corresponding to all respects, except that of hardness, to the bituminous beds of western Pennsylvania , and no doubt originally united with them in continuous sheets over the length and breadth of the State.

The red color on the map forms a narrow zigzag border around this whole anthracite region. This represents the upturned edge of the Catskill formation, No. IX, which underlies the whole; the outward points representing the synclinal basins and the reentering angles the anticlinal arches which separate the several coal fields. A multitude of smaller secondary placations are shown on the county maps, but cannot be given on the scale of the skeleton State map.

The anthracite beds rise steeply to the soil at the edge of each basin, and may be followed in imagination through the air, arching thousands of feet above the spectator's head and descending again into the next basin, to sink 2000 feet or more beneath the level of the sea. It is evident that the aerial arch between two neighboring basins represents that amount of destruction or erosion of the strata underlying the coal, and of the Coal Measures themselves; and a glance at the little map will convince the eyes that, whereas our mining operations are wasting 50 percent of the anthracite coal still left at our command, the frosts and rains of all ages since the Coal Era have wasted for man, before men were created, at least a hundred tons of the original deposit for every ton which they have spared.

The bituminous Coal Measures lie practically in their originally horizontal condition. The waves which traverse Western Pennsylvania and divide it into six grand basins are so gentle that the rocks seldom dip more than 2° or 3°. Consequently, the erosion of bituminous coal has amounted to a waste of only 70 or 80 per cent of the original quantity, and so, whereas our anthracite coal fields must be exhausted in two or three centuries the share which Pennsylvania owns in the great Appalachian coal area- apart from those portions which lie is West Virginia, Ohio, Kentucky, Tennessee, and Alabama- can satisfy the wants of the growing population of the country for several thousand years.

The skeleton State map is colored to show why there is no coal in Middle Pennsylvania, except in eighty square miles of Broad top.

The red Catskill formation, No. IX, crops out from beneath the lower Carboniferous escarpment of the Alleghany mountain- through Somerset, Cambria, Blair, Centre, Clinton and Lycoming counties- as a noble terrace or range of short, knob-like spurs from the front of the mountain. In the long, narrow Bald Eagle valley, at the foot of the mountain, crop up the underlying Chemung, Portage, Genessee, Hamilton, and Marcellus formations which together make No. VIII, the Oriskany No. VII, the Lower Helderberg No. VI, and the Onondaga and Clinton No. V, * 4 at angles increasing from 15° or 20° up to 80° or 90° . The Clinton forms the west side of the Bald Eagle mountain, the two crests of which are made by the two outcrops of the next lower sand-rock formations, the Medina and Oneida, No. IV. The east slope of this mountain is made by the Loraine ( Hudson river ) slate formation No. III; and from beneath this rises, in the Nittany valley, the Trenton , Chazy, and Calciferous limestone formations, No. II. (See cross section along the Little Juniata river on page vi.)

Passing on eastward across the Nittany valley, the limestone (II) turn over and go down again; then III in Tussey mountain; IV males its crest, and V its eastern slope; then VI, VII, and VIII descend; and finally Catskill (IX) reappears sinking eastward into Terrace mountain. Thus from the top beds of the Catskill in Terrace mountain and arch in the air may be drawn, which, if the measured thicknesses of all the abovementioned Devonian and Silurian formations be added together, must have been, along its keystone line, 2560'+6520'+50'+900'+1330'+2900'+900'+6600'=21,760 feet, or a little more than four miles high . That it was a solid arch is shown at its northern and southern ends, where one formation after another from II up to IX sinks slowly beneath the present surface; and that it was much higher than the 21,760 feet above given is shown by the fact that the same conglomerate, red shale, iron-ore, limestone and cola beds are recognizable on the Allegheny mountain and on Broad Top, with opposite dips, twenty miles apart; and since the Pittsburgh coal bed is preserved in Broad Top, although it has been eroded from the Alleghany table-land of Cambria county, we have a right to add to the height of the arch the whole thickness of the three Carboniferous formations overlying the Catskill: i.e. 2500'±; 238', and 240', say 5000'; making the total original height of the arch five miles .

This gigantic rock-wave extended from Sullivan county to Cambria , a distance north-east and south-west of 125 miles. Another, to the south of it, extended from the Luzerne county 165 miles into Fulton county. As the highest part of the first wave was over Nittany valley so the highest part of the second wave was over Kishacoquillis valley in Mifflin county. Between the two were shorter arches of nearly equal height, now marked by Brush and Penn valleys in Centre county. A third great arch commencing in Columbia county extended 120 miles to the Maryland line; but its height over Shade valley in Snyder county, and over Black Log valley in Juniata county, was less than 20,000 feet. A fourth extended from Schuylkill county about 100 miles along the county lines of Snyder, Juniata, and Franklin, into Maryland, being at its greatest height over Path valley, where it was broken so that its western side sank several hundred feet. Between the third and fourth long wave was a short one, in Fulton county, at least 20,000 feet high, but broken lengthwise so that its western side dropped about 8000 feet, making the McConnellsburg fault, where the rocks of No. VII abut against the limestones of No. II. A fifth arch started in Schuylkill county between the two lobes of the fishtail at the west end of the Pottsville coal basin, and extended through Perry, Cumberland, and Franklin counties to the Maryland line, being more than 20,000 feet high in its southern course.

All these long waves had a majestic curvilinear course from east-north-east to south-south-west, and between them lay deep basins of rumpled rocks which still preserve along their middle lines more or less of their Devonian and Silurian formations. But as neither the arches nor the basins were regular and symmetrical in form, but undulated lengthwise, more or less, the lines of outcrop on the present surface which reveal their shape from a curious labyrinth of zigzags throughout from the Juniata region eastward to the anthracite region, the result of their even erosion to the present surface-level has been to confine the blue color (Silurian) mainly to the country west of the Susquehanna, and the red color and overlying white and black to the country east of that river.

The same effect has been produced by the erosion of the corrugated mass as is obtained by planing smooth a log of gnarled wood; and the color-lines upon the map may be considered as imitating the natural graining of a piece of furniture. To produce its full effect upon the eye and to avoid confusion, only the formations Nos. VI, III, and II have been colored; No. VI (the Lower Helderberg) being a thin blue line representing a multitude of low ridges, carrying valuable limestone beds, iron-ore and glass sand; No. III, gray, representing the inside slope of a large number of the mountains of No. IV inclosing the Silurian limestone valleys of No. II. These valleys are colored broadly blue, and serve to mark (a) the ancient summits of the great rock waves, i.e. the extreme heights in the air at which the top strata of the upper coals measures rolled from one great basin over into another, (2) the most fertile and longest settled secluded farming lands of the interior of the State, and (3) its most famous native iron-ore districts, studded as they are with open quarry-mines of brown hematite, deposited in the sink holes and caverns of the present limestone surface.

Beside the five principal rock waves described above as dominating the plicated structure of the middle mountain belt of the State, two others in western and two in eastern Pennsylvania deserve special attention; the former because they sub-divide the bituminous coal region into its 1 st , 2 nd and 3 rd basins and connect our geology with that of West Virginia; the latter because they have determined the remarkable long straight southern edge of the anthracite cola region, and connect our geology with that of northern New Jersey.

The only distinct mountain ranges in western Pennsylvania are those of Laurel Hill along the top of which runs the county line between Somerset and Fayette, and of Chestnut Ridge in Fayette, Westmoreland, and Indiana counties. The broadly rounded crests of these two mountains run NE and SW parallel with each other, and ten miles apart, inclosing between them the Ligonier valley. The rainfall of the highland back of the Allegheny mountain crest in Cambria and northern Somerset collects in the Conemaugh river at Johnstown and cuts a gate-like gap through Chestnut ridge at Blairsville. In like manner the rainfall of southern Somerset collects in Castleman's river, joins the Youghiogheny coming from Virginia , and cuts two similar gaps through the two mountains at Confluence and Connellsville. In the vertical walls of these four gaps, each measuring from 1200 to 1300 feet from mountain top to river bed, the Devonian rocks are seen rising in low arches, overlaid by higher arches of Carboniferous strata (X, XI, XII), and casting off the lowest workable coal beds from their opposites sides into the three great coal basins. Unlike the mountains of middle Pennsylvania, these two mountains have nearly the shape of the anticlinal arches which produced them; but they have nevertheless shared in the general erosion of the State to the extent of more than 2000 feet, or not even the lowest coal bed remains upon their tops for a length of 60 miles north from the Virginia line. In Clearfield county, and further on north-eastward, however, the productive coal measures pass over both these arches from the 1 st into the 2d, and from the 2d into the 3d basin, owing to the decline in height of the arches in that direction, while the surface level of the country is maintained, and even increased. In Virginia these rock waves increase in height going south, and separate the coal basins more widely, allowing the underlying formations to take possession of the present surface. The wonderful straightness of these rock-waves for 150 miles from Fayette to Lycoming county-their parallelism with each other and with the crest of the Allegheny mountain representing the Nittany Valley anticlinal-their equitable height along with such extensive lines, preventing the appearance of the Devonian rocks at the present surface until they reach Clinton county-and their extreme flatness, preventing the formation of such valleys as abound in the middle belt of the State-are all features of great geological importance. And, in discussing the astonishing fact that these two anticlinal waves have only lost at the most 3000 feet of their original height in the same length of time that the Nittany anticlinal, for example, has lost 25,000 feet, we can recognize the extraordinary violence and rapidity of the first stages of the erosion of the State in those districts of it which then stood as high above the sea as do the Alps, Andes, and Himalaya mountains of to-day.

Negro mountain, which enters Somerset from West Virginia, and is cut by Castleman's river, is similar to the two just described; but its rock arch is comparatively low, and passes on into the high flat Allegheny mountain table-land of Cambria county in a direct line with the Nittany Valley anticlinal, of which it seems to be a virtual continuation.

The two rock-waves of eastern Pennsylvania , south of the anthracite region, are shorter and lower than the great anticlinals of middle Pennsylvania , but have a still more important effect in modifying the practical geology of the State. One of them crosses the Delaware river at Walpack bend, traverses Monroe and Carbon counties, crosses the Lehigh river at Weissport, and the Little Schuylkill below Tamaqua, and makes two of the sharp folds in the Pottsville coal basin at New Philadelphia . The other one is confined to southern Schuylkill county, and crosses the Schuylkill river at Schuylkill Haven. They are alike in straightness and direction and in shape, having moderate dips to the south and a very steep or vertical plunge northward. Both of them run at a low angle diagonally to the strike of the outcrops and make a series of zigzags across the valley formations, and of hooks and spurs in the mountain ridges, as the county maps show. But the most important feature common to both is the vertical descent from a vast original height in the air to a profound depth beneath the present surface, of the whole series of Devonian and Carboniferous formations, in the northern leg of each arch. At Schuylkill Haven the pinched top fold of No. VI appears in the bed of the river. From this point up to Pottsville about 15,000 feet of No. VIII, IX, X, XI, and XII, with many hundred feet of the productive coal measures, are cut through by the Schuylkill river. This makes the original height of the arch over Schuylkill Haven about three miles. As the lowest coal bed sinks to more than 2,000 feet beneath Pottsville , the limestone of Schuylkill Haven (No. VI) must lie about 17,000 feet beneath Pottsville ; while the added thicknesses of the Silurian formations from No. VI downward will place the Potsdam sandstone No. I beneath Pottsville at a depth of about 26,000 feet, or nearly five miles.

This vertical plunge of the Paleozoic series of formations from a great height in the air to a great depth beneath the present surface begins in Carbon county and continues through Schuylkill , Lebanon , Dauphin, and Cumberland into Franklin county, although it is not produced by the two arches above described further west than the Swatara river. From the Swatara to the Susquehanna river, and so onward, it is referable to other more southern and parallel folds which operate with even greater effect, for in the gaps of the Susquehanna above Harrisburg all the formations from No. XII down to No. III-strata measuring at least 16,000 feet-are seen thrown over on their faces at an angle of 20° beyond the vertical , and the only suspicion of a break in the arch is suggested by the absence of about 1,000 feet of Nos. V, VI, VII and the lower part of VIII, which may be equally well ascribed to an original absence of the missing strata from the deposits of the ancient sea at the close of the Silurian ages.

As Western Pennsylvania, with its present surface of Carboniferous rocks, is limited in front by the great curved line of the Allegheny mountain wall, 175 miles long, and of an even height of 2000 feet above the sea-so Middle Pennsylvania, with its labyrinth of Devonian and Silurian ridges and valleys, is limited in front by the continuous but minutely crenulated line of the Kittatinny, Blue or North mountain wall, 170 miles long, with its narrow rocky crest, and of an almost perfectly uniform height of 1500 feet above the sea.* 5 Through five gates in this otherwise unbroken wall the rainfall of Eastern New York and Northern and Middle Pennsylvania finds its way to the seaboard, as rivers named in the following order from east to west; the Delaware, the Lehigh, the Schuylkill, the Swatara, and the Susquehanna. The Potomac river makes a distant sicth gap in Maryland , but receives little of the drainage of our State. Middle Pennsylvania is chiefly drained by the Juniata river. The Lehigh river drains only parts of Carbon and Monroe; the Schuylkill drains the rest of the southern anthracite coal basins; the Swatara is scarcely to be named, although its water gap is like the others; the Delaware only drains Wayne, Pike, and Monroe counties in our State; but the Susquehanna spreads its immense water tree over the whole country extending from the Mohawk Valley in New York to Clearfield, Indiana, and Cambria counties in Western Pennsylvania.

In front of the Blue mountain, like a broad moat at the outside foot of a castle wall, lies the Great Valley of the earlier settlers, known by many names, such as the Lehigh, Lebanon or Cumberland valley in Pennsylvania, the Winchester or Shenandoah valley in Virginia, the valleys of East Tennessee, and in the other direction the Kittatinny valley in New Jersey, and the Newburgh valley in New York-extending as it does from the Hudson river to the middle of Alabama, behind the Highlands and South mountains of the north, and the Blue Ridge and the Smoky mountains of the Southern States, which along its course of 700 miles seclude it from the Atlantic seaboard country. Its width in Pennsylvania varies from ten to twenty miles, and is about equally divided into two belts of soil, slate next to the Blue mountain, and limestone next to the South mountain; the former representing the outcrop of from 3000 to 6000 feet of formation No. III, the latter from 2000 to 3000 feet of formation No. II, crimpled into numerous small sharp folds, some of which, however, are large enough to produce alternate strips of II and III upon the map, especially in Franklin county. The gray and blue color belt on the small skeleton State map shows the position, shape and size of the Great Valley ; and at its south-western end indicates with convincing clearness the relationship of its slate and limestone formations to those of the smaller interior valleys of the middle region of the State. Additional proof of their identity and underground continuity is afforded by the iron ore deposits and the Lower Silurian, or Siluro-Cambrian fossils common to both. Species of Murchisonia and Orthoceras are found in the Canoe valley of Huntingdon county, and in the Lehigh valley of Northampton county; and Trenton fossils abound in Nippenose valley of Lycoming county, as along the center line of the Great Valley at Chambersburg and Carlisle .

That the Limestone formation No. II was originally deposited over all south-eastern Pennsylvania is plainly shown by the patches and strips of its blue color on the little skeleton map; for, although it has been eroded from the higher parts of the Reading-Easton hills in Northampton, Lehigh, and Berks counties, it has been preserved in Durham, Saucon and Oley valleys which lie among them; and is seen going down beneath the Mesozoic strata (colored brown) along its northern edge, and rising again to the surface at its southern edge. That it once in [manner overspread the South mountain range west of the Susquehanna river is proved by the preservation of a strip of it in the valley of Mountain creek at Pinegrove furnace. Its outspread southward is shown by its appearance at the surface in Lancaster county, where a triangular piece of the Mesozoic overlying formation has been eroded through; and in Bucks county, where an upthrow seven miles long from the Delaware river south-westward, brings it to the surface, with its fossils and its companion the underlying Potsdam sandstone No. I. The garden of the State is Lancaster county, and why it is the garden of the State is explained by the large area of blue color on the map around the city of Lancaster, extending itself westward through York and Adams counties as the Codorus valley, and eastward into Chester county as the Conestoga valley behind, and the Downingtown valley in front of the (white) area of the Welsh mountain region. There can be no doubt that the Lancaster county limestone formerly covered the whole of northern Chester; and that it was removed from it by gradual erosion before the deposit of Mesozoic sediments; for there is no appearance of the limestone at the present edge of the Mesozoic area along French creek, and there is ample evidence that the Mesozoic itself originally covered the district beyond its present limits. That the limestone formation No. II once overspread southern Lancaster , southern Chester , and Delaware counties and the northern part of the State of Delaware also, is shown by the relics of it left at various places, along parallel lines marked in blue upon the map; and in all such places it is accompanied by its underlying Potsdam sandstone No. I.

The lowest Palaeozoic formation in Pennsylvania , No. I, logically identified with the Potsdam sandstone of northern New York , makes its appearance along the edges of the limestone No. II at the north foot of the Azoic mountain range between Bethlehem and Reading in Lehigh and Berks counties; in Mulbaugh hill on the Lebanon county line; in Chicques ridge on the Susquehanna above Columbia; in the Welsh mountain in northern Chester, and in the North Valley hill which stretches for 60 miles from the heart of Lancaster to the Bucks-Montgomery county line north of the city of Philadelphia. Its only fossil as yet discovered is a Scolithus, but its position next beneath the Calciferous limestone is too well marked to admit of doubt. Formerly it entirely covered the mountain districts north and south of the Schuylkill river, because it still spreads in sheets upon their sides, and in many places makes their summits, lying unconformably upon the gneiss.

The South mountains proper, which separate Cumberland from York , and Franklin from Adams county, do not thus exhibit the fundamental gneiss covered by a coating of Potsdam , but are composed of peculiar sandstone and slate strata several thousand feet thick which occupy the place of the Potsdam in the series but cannot certainly be identified with it. They may be considered the equivalents of the Ocoee and Sewanee strata of East Tennessee .

These rocks when followed south are seen to be cut off by a cross fault along the line of the Chambersburg and Gettysburg turnpike, which has shifted a whole block of the earth crust (north of the pike) westward several miles. South of the fault the sandstone reappears along the western side of the mountain mass, but most of the ground becomes occupied by red and gray schists and porphyries carrying traces of copper, supposed to represent the Huronian system of western Canada. These rocks cross Maryland , and are finely exposed along the Potomac between Harper's Ferry and the Point of Rocks; thence onward they make the Blue Ridge of Virginia, flanked by the sandstones, slates, and conglomerates of so-called Potsdam age. We may consider our South mountain rocks therefore, those lying north of the turnpike fault, as of Cambrian age. At the Carlisle end of the mountain range they seem to be thrown into a series of five anticlinal waves; but along their Cumberland and Franklin county flank they dip eastward instead of westward, i.e., from over the limestone of the valley, instead of towards and under it; so that a long fault must be supposed to follow the foot of the mountain as far south as the turnpike. Sought of the turnpike synclinals and synclinals, vertical dips and offsets t the east complicate the structure and require long investigation.

The geology of southern Adams, York , Lancaster , and Chester is still obscure-a region of metamorphic rocks-mica-schists, chlorite schists, and gneiss of various kinds, interrupted by belts of serpertine and marble, and carrying deposits of Kaolin and chrome iron sand. Where the Susquehanna river crosses Mason and Dixon's line a belt of roofing-slate formation of Northampton and Lehigh county once extended over southern Pennsylvania; and this idea has been carried so far as to suppose that the talcose and micaceous and garnetiferous serpentine-bearing schists which form a wide border to the Chester county limestone valley from York eastward, along the South Valley hill, and across the Schuylkill to Chestnut hill in Philadelphia, instead of being sub-Calciferous, Potsdam, sub-Potsdam, or Cambrian strata, are really metamorphosed Hudson river strata, overlying the limestones of the valley, the top layers of which would then be Trenton beds, turned to white marble.

Immediately south of the belt of these more or less magnesian schists of the South Valley hill, gneiss of much older-looking kind shows itself at the surface, and occupies considerable areas in Delaware county. Similar gneiss occupies the edge of the Mesozoic for several miles west of the Delaware at Trenton . The same older gneiss appears in northern Chester county; is brought up by an arch of the rocks on the Susquehanna river at the mouth of Tocquan creek, fifteen miles below Columbia; forms most part of Mulbaugh hill at the south-east corner of Lebanon county; constitutes the core of the mountain ridges from Reading eastward to the Delaware river; appears in tow isolated hills from beneath the limestone west and east of Allentown in Lehigh county, and in Chestnut hill north of Easton in Northampton county; spreads through northern New Jersey and southern New York in ranges of mountains which make the finest scenery of the Hudson river, and is there recognized by northern geologists as typical Canadian or Laurentian gneiss, the fundamental rock formation of the continent.

Finally, south of the line of this old hornblendic gneiss, in Delaware county, which crosses the Schuylkill river at Conshohocking, and ends in a point at the Wissahickon creek, range the mysterious Philadelphia rocks, which dip at various angles southward towards the Delaware river, seem to be 10,000 or 15,000 feet thick, consist of thin-bedded gneiss and mica-schists with serpentine and garnets, the age of which is as yet not certainly known. Their surface is covered by the older gravels, sands, and brick clays deposited in the Delaware river valley when it was an estuary of the sea, and by the more recent mud of the river at its present level. On the New Jersey side run the outcrops of the lower Cretaceous formation, which, however, occupy the corner of Pennsylvania between Bristol and Trenton .

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After this short survey of the geology of the State as sketched to the eye by the little skeleton map it only remains to give the history and explain the purpose of this hand atlas.

In 1839 I joined the corps of the First Geological Survey; and in 1841, by direction of Prof. H.D. Rogers, compiled the Geological State Map (published by him Edinburgh in 1858) from all the data furnished by the field work of the corps during the six preceding years. That map has now been under the closest critical examination by my colleagues of the Second Geological Survey, since 1874, and has been found to be in all essential particulars a correct representation of the topographical and geological structure of the State, but both deficient and defective in a multitude of details, which have now been in great part supplied and defined in the colored geological county maps published during the last nine years with the Reports of Progress of the Second Survey, as it advanced in its reexamination of the whole ground.

Of the sixty-seven counties of the State, fifty-seven such maps, on a uniform scale of two miles to an inch, have been published; five others have been printed, and await publication; five others remain to be prepared. Were a new State map compiled from these county maps, it would be a great improvement on my map of 1841 (1858) in the case of the counties lying between the Allegheny mountain and the Ohio line; while it would be little more than a reproduction of the old map as to Middle and Eastern Pennsylvania, so excellently were these studies and portrayed by the geologists of the First Survey; and this, not merely in a geological sense, but in regard to geographical correctness; for in the interval between the two surveys, and interval of thirty-three years, private geographical surveys were made, and private county maps and atlases of township maps published, covering the whole area of the State. These have been used by the geologists of the Second Survey, and constituted an improved basis for their geological mapping. But the errors discovered in these county and township maps in following the outcrops of the geological formations from stream to stream and from road to road are innumerable, and, in a multitude of places, large. As the township survey plottings were more or less distorted in the process of forcing them together within the boundaries of each county map, and as the county boundaries have never been properly located in the area of the State, it is impossible to compile a State map without employing the same vicious method of forcing the counties together by distorting their boundary lines, and superposing thus a new network of errors upon the old one. And this state of things respecting the geography of Pennsylvania must continue to stand in the way until the State has been properly triangulated and new township and county surveys made, starting from the corners or the triangles,-a costly and tedious undertaking, but one worthy of a great and wealthy Commonwealth.

Anticipating the difficulties in the progress of the Survey towards the production of a geographically correct State map, the Board of Commissioners approved my plan of providing a small county atlas of the State for the use of geological field-workers, prospectors, engineers, and travelers in general, by reproducing on a much smaller scale, six miles to an inch , the geological county maps as fast at they were prepared for the Reports of the Progress of the Survey. This I began to do in 1875, and colored by hand one county after another, in successive years, completing in the spring of 1884 the whole series, including counties of which maps on the two-mile scale had not been made. This atlas is therefore a virtual second edition of my map of 1841 (1858) improved by the Second Survey, but not presented on a single sheet.

The following acknowledgements are due:

For Greene and Washington, to Prof. Stevenson and Prof. White; Fayette, Westmoreland, and Allegheny, to Prof. Stevenson; Beaver, Lawrence , Mercer, Crawford, Erie , and parts of Allegheny and Butler , to Prof. White; Clarion and northern Butler , to Dr. Chance; Venango and Warren, to Mr. Carll; McKean, Forest, Elk, and Cameron, to Mr. Ashburner and Mr. Sheafer; Jefferson . Armstrong , Indiana , and Somerset , to Mr. W. G. Platt; Cambria, to Mr. F. Platt and W. G. Platt; Clearfield and Clinton , to Dr. Chance; Centre, to Mr, d'Invilliers; Blair to Mr. F. Platt and Mr. Sanders; Bedford and Fulton, to Prof. Stevenson; Huntingdon, Mifflin, Juniata, to Dr. Dewees, Mr. Ashburner, and Mr. Billin; Union and Snyder, to Mr. Billin; Lycoming, Sullivan, and Wyoming, to Mr. F. Platt; Potter, Tioga, and Bradford, to Mr. Sherwood and Mr. F. Platt; Susquehanna, Wayne, Pike, Monroe, and parts of Carbon, Lackawanna, Luzerne, Columbia, Montour, and Northumberland, to Prof. White; Perry, to Mr. Dewees and Prof. Claypole; Franklin, Cumberland, Dauphin, Lebanon, to Mr. Sanders; Berks, to Mr. d'Invilliers, Mr. Sanders, Prof. Prime, and Mr. Kent; Lehigh and Northampton, to Prof. Prime, Mr. Berlin, and Mr. Sanders; Adams and York, to Prof. Frazer and Mr. Lehman; Lancaster and Chester, to Prof. Frazer; Delaware and parts of Philadelphia, Montgomery, and Bucks, to Mr. Hall; Schuylkill, Lehigh, and parts of neighboring counties, to Mr. Ashburner and his assistants; and various parts of the State at large and in details to Mr. E. B. Harden and Mr. O. B. Harden.

Where their work on the First Survey stood good the same acknowledgement should be made for the southern and middle anthracite region to Dr. Whelpley, for the Juniata region to Dr. Henderson, for the northern anthracite region and the mountains between the West Branch and the Juniata to Mr. McKinley. But the early work of Dr. Jackson, Dr. Hodge, Mr. McKinney, Mr. Hall, and myself in the northern and western regions of the State have been entirely superseded by that of the Second Survey, as assigned in detail above.

The elaborate topographical work of the Second Survey in McKean, Blair, Huntingdon, Mifflin, Centre, Snyder, Berks, Lehigh, and Northampton counties (published in the atlases to Reports R, T, and D, and awaiting publication in S) is represented on the maps of those counties in this atlas; but the topographical work done in the South mountains was not in form for used when the maps of York, Adams, Franklin, and Cumberland were colored. The map of Perry county was printed before the revision of that county (with special study of its faults and trap-dykes) was made by Prof. Claypole, as published in Report F. The maps of Schuylkill, Carbon, Lehigh, Luzerne, Lackawanna , Columbia , and Northumberland represent the anthracite basins only approximately, but their accurate delineation can be learned by consulting the large sheets of the anthracite survey.

 

 

ENDNOTES

* 1 See the special map of Philadelphia , and Report C 6 .

* 2 See the maps in Report Z.

* 3 The black spot in Huntingdon county represents the entire 80 square miles of Broad Top Coal Measures. The patch of Pittsburgh coal would be represented by the finest needle prick, and must be imagined to lie in the center of the black spot.

* 4 The Oriskany is the base of the Devonian and the top of the Silurian systems.

* 5 If the measurement be made by following the crest around the hooks, and up and down Path valley, it will amount to more than 260 miles in Pennsylvania alone.

From: A geological hand atlas of the sixty-seven counties of Pennsylvania :embodying the results of the field work of the survey, from 1874 to 1884. By J. P. Lesley. (Report of progress (Geological Survey of Pennsylvania), v. X ) Harrisburg, PA : Board of commissioners for the second geological survey, 1885.

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