84 PROCEEDINIJS OF THE ACADEMY OE [1890. 



1000 feet northwest of Thorp's Lane and the steatite belt, is a bed of 

 porphyritic granite, or granulite, clearly interstratified in the 

 schists. Just below Thorj)'s Lane the peculiar contorted structure 

 is well shown. 



It is in these Chestnut Hill schists that the soapstoue or steatite 

 belt is found, and these rocks bound it on both sides, though at the 

 Soapstone quarry on the northeast bank of the Schuylkill, at con- 

 siderable depth, there was a hornblendic gneiss as the southeast wall, 

 which, next to the steatite, passed gradually into chlorite. 



It is to these schists that I understand jNIr. Hall to believe^ that 

 all the serpentines of the district are confined (except the Cresheim 

 Creek, the Flushing and the steatite of the northerly Radnor belt 

 near Gulf Mills) and it is these which he continues along the line of 

 the Pennsylvania Railroad from the Lafayette belt at'Rosemont to 

 the Radnor belt at Radnor station so as to include them both. But 

 I have shown that this region is Laurentian and that schists are 

 absent.^ 



JNIr. Hall suggests that Mill creek may be the boundary between 

 the Manayunk and Chestnut Hill schists, but it is remarkable that 

 though ]\Iill creek rises in these soft schists and the slopes of the 

 Laurentian bordering them to the northwest, it crosses the border 

 into the harder rocks and has cut through them a deep narrow val- 

 ley to the river, close to, but S. E. of, the border line of the soft 

 schists. 



In the Chestnut Hill schists, west of the Schuylkill, are strata 

 of white quartz becoming rusty on exposure. Seen occasionally in 

 place, as on the Roberts road, and in the Railroad cut at Bryn Mawr, 

 they are more frequently the only rock in the soil, and sometimes 

 are very abundant, affording a guide to these schists, where they 

 have decomposed out of sight, but not a certain guide, for in some 

 places, as north of Morton, they seem to overlie the Manayunk 

 schists. Perhaps the Chestnut Hill schists did overlie, and have 

 been eroded, the hard quartzite only being left. This quartz occurs 

 in large quantity at the narrow exposure of the schists in Marple. 



Southwestward of the Schuylkill the divisions given above are not 

 so distinct. The section on Cobb's Creek shows the porphyritic 

 gneiss within a quarter of a mile of the soft garnetiferous schists, but 

 here rocks, apparently of the jNIanayunk series, but nuich resembling 



1 C« p. 3. 



2 2nd Geol. Survey of Pa., An. Rep. 1888, Part iv, p. 1573. 



