sandstones forming prominent ridges, having a much less dip and covering greater areas. There are several cynclinals and anticlinals, the axes of which form an obtuse angle to the line of strike of Edge Hill. These folds die down very rapidly to the eastward, but are marked by slight irregularities in the line of strike of Edge Hill.

I have been long inclined to consider the elevations of sandstone along the north flank of the limestone valley, as having been caused by folds and faults, but I do not see any proofs of faulting. But there is evidence of unconformity between the two groups, as at Spring Mill, where the limestone overrides the upturned Potsdam; at a point west of the second crossing of Sandy Run by the North Penn. R. R., just south of Fort Washington, the limestone occurs in a bay in the sandstone, having a slighter dip. Just south of Mogectown, cast of the Schuylkill river, near Norristown, the limestones rest unconformably upon the Potsdam, to all appearances.

The anticlinal ridges of Potsdam extending diagonally into the valley, are flanked on both sides by limestone, and, in some cases, disappear below the limestone which encircles it; but in the most eastern fold I doubt whether the limestone is connected.

The marble is confined to the south side of the valley. The dip of the limestone being to the south, it would place them at or near the top. Marble quarries are found in Chester county, close along the southern margin of the limestone valley, and in close proximity to the slates. In Montgomery county the same rule holds good, and very shortly after the disappearance of the slates to the east of the Schuylkill, we find the marble is missing as well.

The rocks rise rapidly to the eastward. The slates of the South Valley Hill are in regular succession upon the limestone. This may be seen where the South Valley Hill ends at Gulf Mills.

Gulf Mills to the Schuylkill river.

N.

The structure is clearly proven by the succession of rocks, which is marble, bastard marble, shale, on the north side of the synclinal and a double repetition on the anticlinal at Gulf Mills. The cove made by the erosion of the anticlinal is just west of Gulf Mills, west of Conshohocken. The transition shaly limestones are repeated several times by minor contortions, and extend over a wide area from

The alternations from the limestone into the slates is everywhere visible along the southern margin of the Chester valley.

The slates are traversed by a trap dyke, which crosses the Schuylkill at Conshohocken. I do not think it marks the line of any disturbance of im portance. The synclinal and anticlinal of the slates do not cross the Schuylkill river. The slates flanking the marbles east of the Schuylkill lie in a monoclinal, pitching to the south against the Potsdam, which was

1880.]

[Hall.

upturned along the line from Spring Mill to Edge Hill P. O., probably prior to their deposition. This is evident from the fact that the limestones lap over the rocks of the second group at West Conshohocken.

It may be that the great unconformity of the measures west of the Schuylkill river cannot be explained without a fault along the line of junction. The southern margin of the slates of the South Valley Hill is somewhat irregular, they come in contact with the rocks of the second group as far west as the east branch of the Brandywine creek, or that neighborhood, where they are succeeded and overlaid by micaceous garnetiferous rocks with limestone of the sixth group.

The southern boundary of the South Valley Hill slates or Lower Hudson river slates, is about on a line from West Conshohocken to a point about four miles north-west of West Chester, and gradually diverges from the line of the Chester valley as we proceed westward. This divergence is caused partially by the flattening of the measures, and partially by their increased thickness.

The sixth group which flanks the South Valley Hill rocks on the south and overlies them, increase rapidly in thickness to the west of the Brandywine creek. This group lies upon the rocks of the second group and encircles the western end of that area. The East Branch of the Brandywine creek cuts along close to the line of junction, between these groups. The schists in places are found on both sides of the creek, while at other points the syenitic rocks of the second group extend west of the line. The limestones of this group are well exposed in the neighborhood of Doe Run. The most easterly exposures are at Caleb Cope's and Copesville on the Brandywine; also at Brinton's Bridge on the Brandywine, and two localities east of Brandywine creek, one in Birmingham and the other in Thornbury township.

The locality at Caleb Cope's place, north-west of West Chester, is a similar deposit to the one at Brinton's Bridge, a thin bed of impure crystalline limestone between beds of schistose rock.

The first locality is on a line with the locality at Cope's Mill, and as can be seen by the map, is also on a line with a number of limestone quarries west of it. I have satisfied myself by walking over the ground that they all belong to one horizon, and if not absolutely continuous they are only broken by the thinning out of the limestone itself. The locality at Brinton's Bridge is on a line with the quarrjes of East Marlborough and London Grove townships, and although,it is a small bed, it is flanked on both sides by the same rocks which flank the heavier beds further west. The fact of it being a thin bed bears me out in the assertion that all these beds thin rapidly towards the east.

I am not prepared to say whether or not there is more than one horizon of limestone in this portion of Chester county. I am forced to the conclusion as to its superposition to the South Valley Hill slates, as the succession is clearly seen along the East Branch of the Brandywine Creek.

The rocks of this group seem to vary considerably, and it is not improbable that they may have to be subdivided, as I have included all the

schistose rocks with limestone from the South Valley Hill slates along the East Branch of the Brandywine to Chadd's Ford or the Maryland line, and west at least as far as Avondale, Chester county. From their position above the slates of the South Valley Hill, which are Hudson river, they belong to a limestone group above the Hudson river group. Inasmuch as no fossils have been found as yet, it is difficult to assign them to any particular age, but I am inclined to think that they may be Silurian and possibly Helderberg. There may be an unconformity between these schists and sandstones and the slates below, but as yet I have not been able to determine the area of the upper group accurately, and before this is done it is impossible to state decidedly what the relation is.

In Eastern New York, south-west of Albany, we find the Hudson river shales and sandstones overlaid by the Niagara in thin beds of concretionary limestone, often not recognizable, followed by the Helderberg limestone, the Oneida conglomerate, Medina sandstone, the Clinton group, and Onondaga shales, all having died out east and north before reaching the Helderberg escarpment. May we not have similar structure here?

The limestones of the Chester Valley extend in an almost straight line from the Schuylkill river to the neighborhood of Quarryville, in Lancaster county, where the straight valley ends and connects with the great limestone valley of Lancaster county at Camargo P. O.

At Camargo P. O., according to Prof. Frazer's map, a tongue of slates connects across the limestone and is colored the same as an area of rock north of the limestone valley, extending to the county line south and east of the Gap P. O.

The limestone valley from the Schuylkill to Quarryville or thereabouts is a monoclinal, the beds all pitching to the southward, followed by Chloritic schists, Hydromica schists and Mica schists, which overlie the limestones. This is clearly demonstrated at the eastern end of the South Valley Hill, which is formed by these slates. The marbles, which are always confined to the southern edge of the valley, mark the horizon of the Trenton limestone with its alternations of slate and slaty limestone, passing by alternations into shale and slate of the South Valley Hill or Hudson river group. Just north of Gulf Mill a synclinal of the slates dies out, and at Gulf Mill we have a double repetition of the alternations of impure limestone and slate found flanking the synclinal on the north, which proves Gulf Mill to be on an anticlinal axis. (See wood cut.)

The dips, as a rule, are pretty high towards the Schuylkill river, varying from 50 to 85°. But as we proceed westward they become somewhat less, which may in part account for the widening of the slate area before spoken of, but there is a marked thickening in the beds to the southwestward, which also must be taken into consideration.

Owing to the short time I have for the preparation of this paper, I will proceed to carry these determinations of horizons south-westward.

At Camargo P. O., we would have an anticlinal of slates overlying the limestones, which anticlinal would be on a line of the axis of the Tocquan creek anticlinal recognized by Prof. Frazer on the Susquehanna.

1880.]

[Hall.

The lower portion of Pequea creek flows along an anticlinal in which the limestone is exposed along the creek to the neighborhood of Marticville P. O. A tongue of slates extends eastward between the Pequea and Conestoga creeks as far as Willow Street P. O., which is on a line of a synclinal axis shown to end at Compassville, and along which the Pequea creek flows from Compassville to Wheatland Mills P. O. An anticlinal having its axis about on a line between Petersville P. O. and the mouth of Conestoga creek. A synclinal of Chloritic slates ending somewhere near Indian town. An anticlinal exposing the limestone, extending from Prospect Furnace P. O., in York county, to Lancaster.

A synclinal of slates extending from a point south-east of Montville P. O. to Washington Manor P. O., and crossing the river, is evident from the distribution of slates in York county, beyond the line of the Peach Bottom R. R. Lastly, we have a monoclinal flanking the Chickis Potsdam, extending from Hempfield, which is at the extreme eastern end of the Potsdam anticlinal through Columbia in Lancaster county, and Wrightsville, York, Springforge to Hanover in York county. North of York the limestone is not continuous across to the northern arm of the anticlinal, which is principally covered by the Trias, but has a considerable exposure in Lancaster county. The slates, therefore, south-east of the limestones of Chester county, are of Hudson river age.

The slates of York and Lancaster counties, which includes the roofing slates of Peach Bottom, are a continuation of those of Chester county, which flank the limestone on the south, and are referable to the same group.

The serpentines of Radnor township, Delaware county, and those of Easttown, Willistown, East and West Goshen, are undoubtedly altered beds of the South Valley Hill slates, or Hudson river slates. They lie unconformably upon the syenitic rocks of the second group.

The probability is that all the serpentines of Chester county will be found to belong to the Hudson river group, and are possibly pretty nearly on the same horizon as the limestones of West Bradford, Merlin, east and west Marlborough, London Grove, Kennet, and other townships of southern Chester county, although I have spoken of this limestone group as being possibly of Helderberg age. The serpentines of southern Lancaster county are undoubtedly altered beds of the Hudson river group, and from their relative position to the roofing slates of Peach Bottom, would be in their proper place.

Dr. T. Sterry Hunt insists that the serpentines of the Schuylkill are below the Philadelphia schists. If they are, the structure would be even more simple than otherwise. Placing them below the Philadelphia schists they would be on a horizon with the serpentine beds of Chester county, and these Philadelphia rocks equivalent to those which they resemble in southern Chester county; but if the serpentines of Montgomery and Delaware counties are above the Philadelphia series, they necessarily belong to a later age than those of the Hudson river group.

At present I am inclined to place these serpentines above the PhiladelPROC. AMER. PHILOS. SOC. XVIII. 105. 3E. PRINTED MARCH 2, 1880.

phia rocks, and, by so doing, assign the Philadelphia series to a higher group than the Hudson river.

The relation of the Philadelphia schists to the schists of the sixth group is not fully determined, but they bear a great resemblance to them, and in many respects are identical. The syenitic rocks of Delaware county which belong to the second group, cut off the connection between them.

To all appearances the serpentine belts, which are visible on the Schuylkill river at Lafayette station, Montgomery county, and at a point just north of them, are above the mica schists of Philadelphia. The southern belt extends in an almost unbroken line from Chestnut Hill, Philadelphia, to Bryn Mawr, in Montgomery county. A less prominent belt extends from the Schuylkill river to the neighborhood of Rosemont station, on the Pennsylvania R. R., in a parallel line to the first belt.

The serpentines of Delaware county are on a general line of strike with these belts, and without doubt represent the same horizons.

There is no evidence in this section of the Oneida conglomerate or Medina sandstone, as far as I can observe, but there are sandstones in the south-eastern portion of Chester county I have classified with the sixth group, which may prove to belong to a sandstone formation succeeding the Hudson river slates.

There is no doubt that magnesian beds may be altered into serpentines wherever they may be, and the mere fact of serpentine existing at any place is not proof of a given horizon, but it is in all probability confined to definite horizons within limited areas.

The whole question of structure would be easily solved could we prove what is everywhere indicated, viz, a gradual subsidence of the formations north-west of the line of junction between the South Valley Hill slates and the syenitic rock of the second group, which change in level, at the close of the Hudson river group, allowed the upper magnesian beds of that group to be deposited over the edges of and encircle the second group.

The evident nonconformity between the Philadelphia schists and the