518 Geological Society of Pennsylvania. 



has been found, except in boulders and veins of other rocks. 

 This rock is the general but not continuous Atlantic boundary 

 of the United States. When Ashton street, in Philadelphia, 

 was regulated, a boulder of granite, weighing several tons, was 

 found in the gravel. The felspar was flesh-coloured, sp. gr. 2.727. 

 Wliere it is found in veins, the felspar is white, and the sp. gr. 

 2.70 1 . The river boulders vary in their constituents more than 

 the inland ones. Mr. B. has found Porphyry with helyotrope 

 among the first. The gneiss at Fairmount water-works con- 

 tains white beryls, and schorl in granite veins, sp. gr. 2.620. 

 The gneiss re-appears at the falls of Schuylkill, five miles from 

 Philadelphia. The State Penitentiary, in Coates street, is built 

 of this rock : it contains, in granite veins, green beryls, phos- 

 phate of lime, scapolite, &,c. In some places the gneiss is en- 

 tirely disintegrated, lying in extensive loose sandy beds. Mica 

 slate is quarried near Lemon hill, and in Broad street ; it is 

 well displayed at the mill-dam upon old Fourth street, sp. gr. 

 2.038 to 2.712. This rock contains granite veins with white 

 beryls, and hexagonal crystals of mica. The beryls, when first 

 detached, are soft and friable, but become gradually harder. 

 Near Bartram's garden, the mica slate is found in disintegrated 

 and sandy beds. At Chesnut hill, the mica slate is nearly verti- 

 cal. From Manayunk to the Plymouth dam, it becomes com- 

 pact, and contains beryls, zeolite, cyanite, schorl, phosphate of 

 lime, &c. The first hornblende rocks which appear near Phila- 

 delphia, are near Fairmount water-works ; but further north, 

 they may be traced from the Delaware to the west bank of the 

 Schuylkill. They appear on the Delaware about a mile beyond 

 the village of Frankford, at the forks of the Bristol and Bustleton 

 turnpikes, and near Second street road. At Manayunk they 

 pass into hornblende slate. Actinolite is found subordinate to 

 this formation. Mr. B. succeeded in detaching a specimen of a 

 very tough compact hornblende, on Second street road, near 

 Frankford creek, and on examining it a few days afterwards, 

 found the fractured surface spangled over with crystals of acty- 

 nolite, similar to others he had observed on the exterior of the 

 rock. At Streaper's hill, on the Ridge turnpike road, the horn- 

 blende rocks again appear. In a ravine at the side of the road, 

 about 1 1 miles from Philadelphia, large boulders of hornblende 

 arc lying on clay slate ; others are found on the surface further 



