66 



OUTLINES OF GEOLOGY. 



the greatest abundance. Among the localities may be men- 

 tioned those of Kingsbridge, Sparta, and Eastchester. From 

 the latter were quarried the splendid pillars of the late Ex- 

 change, and the obelisk erected to the memory of Emmett, 

 in single blocks between 30 and 40 feet in length. It is said 

 that pieces of 100 feet in length might be readily extracted 

 from the same quarry. The neighbourhood of Philadelphia 

 furnishes similar marbles, but the quarry seems to have been 

 exhausted, as the blocks recently extracted are veined and 

 incline to a gray colour. A still more beautiful variety is 

 found at Stockbridge, in Massachusetts, some of which is of 

 so fine a grain as to rival the Parian. The firmer layers 

 of these rocks, when free from pyrites, seem to be almost 

 indestructible by the weather. 



The gneiss is valuable as a rude building and paving stone, 

 and with the gneisiform rocks is often worked as a building 

 stone under the name of granite. This, although harder, is 

 rarely as durable as the white marble, for the felspar it con- 

 tains is liable to decomposition, and causes the rock to de- 

 squamate. 



156. The crystaline rocks of the inferior order have been 

 found beneath every other formation, from those ef the mo- 

 dern epoch to the slates ; and although in many cases the 

 thickness of superinposed formations is such as to prevent 

 their being penetrated, it has been inferred, that these crys- 

 taline rocks envelop the whole globe, and serve as the basis 

 of all the other formations. Beneath the gneiss no other 

 rock has been found, except such as are granitoid, and it is 

 by no means clear that even rocks of this description are 

 always inferior to it in position. 



157. In referring back to the descriptions of the several 

 stratified formations, it will be seen that no more than three 

 can be considered as enveloping, or equally extensive with 

 the surface of the continents. These are : the humus, the 



