schistose rocks with limestone from the South Valley Hill slates along the East Branch of the Brandy wine 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 wltat 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 isamonoclinal, the beds all pitching to the southward, followed by Chloride 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 spokeD 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.

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 slateii, 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 Philadel

PROC. 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.

LIMESTONE.

MICA SCHISTS.

SERPENTINES.

GARNETIFEROUS SCHISTS.

HYDROMICA SLATES.

CHLORITIC SLATES.

III.

Alternations Of Slate
And Limestone.

MARBLE.

SLATE.
DOLOMITES.
I. POTSDAM Sandstone.

QUABTZITE.

SYENITIC AND
GRANITIC ROCKS.

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