540 PROCEEDINGS OF THE ACADEMY OF [1895. 



December 3. 



The President, General Isaac J. Wistar, in the Chair. 

 Forty- six persons present. 



Trap Dykes in Chester County, Pennsylvania. — Theo. D. Kand 

 stated that two important trap dykes in Chester County had seemed 

 almost to have escaped notice. One of these, a peculiar porphyry, 

 described by Mr. Goldsmith as containing the variety of silica, 

 vestan, is best shown in Williams' quarry, near Aldham, on the 

 Phoenixville branch ot the Pennsylvania Railroad. It extends 

 thence in a northeastwardly direction with a width of about 100 ft. 

 It is not apparent to the westward, but a rock which Mr. Goldsmith 

 has pronounced identical occurs near Barneston Station on the 

 Waynesburg branch of the Pennsylvania Railroad. Some masses 

 of this rock have a loud and clear ring when struck, as shown in 

 specimens exhibited to the Academy by Mr. Borden. 



The other dyke forms a bluff on the right bank of the Brandy- 

 wine, almost in Downingtown, and perhaps 500 ft. north of the bridge 

 which carries the Lancaster Turnpike over the creek. It is again 

 exposed on the Pennsylvania Railroad just above the station, and 

 again much more largely a quarter of a mile further west, where on 

 the south side it is exposed for a depth of nearly thirty feet. Thence 

 it may be traced by fragments up the South Valley hill for prob- 

 ably five hundred feet in a general S. S. W. direction. About two 

 miles southwest, near the source of Broad Run and nearly north of 

 Romansville, it again appears, but no outcrop between could be 

 found. In the same direction, about two miles further, it appears at 

 Mortonville, on the right bank of the west branch of the Brandy- 

 wine. Three-quarters of a mile further, at the crossing of Buck 

 Run by the old road from Mortonville to Doe Run, it is exposed in 

 large loose masses. A mile beyond it appears south of Doe Run 

 village and thence southwestward is almost or quite continuous, 

 being in vast quantity southwest of the Marlborough Hall School- 

 house. It is crossed by the Pomeroy and Newark Railroad at the 

 southwestern part of the S curve near the source of the south branch 

 of Doe Run. Here it looks almost like cobblestones, to so great a 

 degree has boulder decomposition occurred. Three miles beyond, it 

 was found by Mr. Harry Wilson, of Green Tree, to whom my ac- 

 knowledgments are due for much aid in tracing this dyke. Fur- 

 ther southwest in Peuu and Lower Oxford townships are several 

 trap outcrops, some of them included by Prof. H, Carvill Lewis 



