184 TROCEEDINGS OF THE ACADEMY OF [1892. 



The most noted marble ({iiarry is at Marble Hall, Montgomery 

 County, on the Ridge Road about fourteen miles from Philadelphia. 

 This yielded fine statuary marble and was wrought as an open 

 quarry to a depth, I am informed, of over three hundred feet; near 

 by lignite and iron pyrites occur. 



It has been suggested that this marble is due to the alteration of 

 the limestone by the Conshohockeu trap dyke which is near its north- 

 westerly side. Inasmuch as there is ordinary limestone between 

 the marble and the dyke and the marble occurs also at a distance 

 from the dyke as at Potts and Henderson's quarries, this view seems 

 untenable. At Conshohockeu, in the cut of the Schuylkill Valley 

 Railroad, the trap dyke was almost if not quite in contact with the 

 limestone, which showed no change from the ordinary limestone of 

 the region. 



Northeast of this was a suialler quarry in an inferior marble, and 

 in this was once found a mass of barite of many tons weight almost 

 indistinguishable from the marble except by its weight. It is a 

 tradition that it was supposed to be marble until the hoisting tackle 

 having parted three times successively while attempting to lift a not 

 unusually large block, an investigation showed that it weighed one 

 and one-third times the same bulk of marble. Carbonate of 

 strontia Avas reported from this vicinity, but I think it a mistake. 



Mr. Jefferis informs me that in 1837 fine crystals of dog-tooth 

 spar were found in quantity at Marble Hall. 



East of the Schuylkill between the limestone and the Laurentian, 

 is a stratum of Cambrian sandstone forming during most of its course 

 a prominent ridge, especially near Edge Hill Station, on the 

 North Pennsylvania Railroad. In this rock, hematite occurs 

 abundantly in brilliant cleavable masses, very rarely crystallized. 

 It is slightly titaniferous, and was found by Mr. Edward D. 

 Drown, upon land belonging to him near Weldou, to contain rutile 

 in interesting specimens. 



On the right bank of the Schuylkill, just above Conshohockeu, 

 Montgomery County, a large quarry known as Bullock's has been 

 wrought for many years in a limestone much mixed with mica and 

 graphite. The rock is tough and durable and is favorably situated 

 for quarrying. It cleaves in one direction with facility and in the 

 others it is divided by joints many feet apart, enabling stones of 

 almost any size to be readily procured. This renders it the pre- 

 ferred stone in Philadelphia for heavy foundations and it has been 



