76 PROCEEDINGS OF THE ACADEMY OF [1890- 



NOTES ON THE GENESIS AND HORIZONS OF THE SERPENTINES OF 

 SOUTHEASTERN PENNSYLVANIA. 



BY THEODORE D. RAND. 



Except the serpentine and steatite of Chestnut Hill, north of 

 Easton, Pa., the outcrops considered in these notes lie not far from 

 the Laurentian anticlinal which enters the State at Morrisville, 

 opposite Trenton, N. J., and extends in an almost unbroken belt to 

 the Maryland line. In width it varies. Generally narrow north- 

 east of the Schuylkill, but usually a prominent ridge, it sinks near 

 Abingdon to appear again southwest of Jenkintown for two or 

 three miles. At Chestnut Hill and the Wissahickon it again disap- 

 pears for a short distance, rising again to great elevation (350- 

 400 feet) with a width of nearly a mile at the Schuylkill. 



Mr. Hall's map show's the Laurentian crossing the Wissahickon 

 on the southwest flank of Chestnut Hill, Philadelphia, northwest of 

 Thorp's lane, and Dr. Hunt^ speaks of the schists appearing on the 

 north side of the narrow Laurentian belt at Chestnut Hill. I 

 have been unable to find a trace of Laurentian along the 

 Wissahickon or at Chestnut Hill except on Pajier Mill Lane on 

 the northwest flank of Chestnut Hill. 



Southwest of the Schuylkill it preserves its elevation, its summits 

 nearly or quite 500 feet above tide, and it widens, until at Darby Creek 

 it is at least three miles wide, and north and south of Newtown Square 

 at least six, though in this longitude there may be included some 

 schist areas ; but the margins at Devon Inn, and south of Berwyu, on 

 the north, and Sycamore Mills on the south, are well defined and 

 exhibit the steep slopes with abundant outcrops so characteristic of 

 this group. Southwest of this the northerly margin trends about 

 S. S. W., passing West Chester one mile north of the Court House, 

 while the southerly margin, defined between Ridley and Chester 

 Creeks along Dismal Run by a px-ecipitous hill of 200 to 300 feet ele- 

 vation, becomes ill defined, but as observed by Prof Rogers- probably 

 takes a direction nearly west through or near Howellville, Delaware 

 Co., and Westtown, in Chester Co., the belt narrowing to less than 

 three miles. 



1 Min. Fhys. and Phys : p. 437. 



2 1st Geol. Survey of Pa., Vol. l,p. 77. 



