48 TRANSACTIONS OF THE [JAN. 7, 
tended so as to connect with the Philadelphia rocks. The 
section does not claim to be an accurate geological section. We 
have indeed no data yet for that, but it simply shows the order 
of the rocks, and to some extent the dips, and there is alsoa 
hint of the possible structure. ’ 
The backbone of the region is the range of Laurentian gneiss 
which extends from the Delaware at Trenton south-eastward. 
To the north of this lies the great Chester-Montgomery Valley, 
of Cambrian and probably Trenton limestone, flanked on the 
north by the elevated range of Potsdam—the North Valley Hill. 
Between the Laurentian and the Chester Valley lies a range, 
uniformly of hydro-mica schist, from one to three miles broad, 
known as the South Valley Hill, otherwise Bethel Hill. Between 
the Laurentian and this South Valley Hill is a narrow valley 
locally known as Cream Valley; this is not over a half to a 
quarter of a mile in width, and the exposures of rocks are few 
and far between, but most interesting. 
In Cream Valley, north of the Laurentian, occurs at several 
localities a stratum of feldspathic sandstone with micaceous 
partings, and more or less rhomboidal fracture, full of minute 
tourmalines. This rock seems to be absolutely identical with 
the eurite of Spring Milland Barren Hill, which lie to the north- 
eastward, in the same relation to the Laurentian, and which 
has been identified as Potsdam sandstone. North-west of this. 
Potsdam sandstone is a limestone, two hundred or three hundred 
feet wide at the mouth of the valley near the Schuylkill River, 
but only about fifty feet wide, three miles from that stream; 
this limestone resembles very closely some of that of the Chester 
Valley (Cambrian), being crystalline, and containing in some 
layers crystals of pyrites—elongated cubes; the exposures of 
this limestone are very few and very poor. 
In the floor of this valley occurs a mica-schist, at times filled 
with garnets, and occasionally containing staurolite; at the 
Schuylkill and also N. W. of West Chester (about twenty miles 
S. W.), the limestone appears to be within this schist. At 
Brooks’ quarry, near Radnor Station, the rocks bounding the 
limestone are schistose feldspathic gneiss, but I think this is a 
local modification of the garnetiferous schist. 
In Cream Valley there are also two belts of serpentine. That 
nearest the Laurentian forms an extensive belt beginning a half 
mile east of Radnor Station, and extending thence south-west- 
ward ina series of outcrops of echelon structure (not continu- 
ous, but with no very long breaks), some twenty miles to the 
Brandywine ; this is close to the Laurentian ; indeed at Radnor 
it appears to be entirely within the Laurentian ; but in many 
places there is a garnetiferous schist associated with it. This 
