(leohxjji (if J\icJiiiiuii(l ('oitiil II. X. Y. 16? 



formation verv improbable ; hence, by tliis method of reasoning, 

 we have notJiing bnt limestone to refer the original condition of 

 the.se strata to. 



Foiirth. — In a<lditi()n to these negative consideratiojis. we 

 haye the direct })ositiye evidence that strata of raagnesian lime- 

 stone gradually passing into ser[)entine have been observed (see 

 Jukes' Manual of Geology, page 167.), and that the presence of 

 lime-minerals in the rock may be regarded as indicative of the 

 former presence of greater quantities of calcic carbonate, which 

 has been removed by the dissolving action of the metamorphos- 

 ing waters, which doubtless held carbonic acid and silica in 

 solution. 



We may then outline the jirobable origin of these rocks in the 

 foUoAving manner : — The strata now consisting of serpentine 

 were deposited as highly magnesian limestones ; by metamor- 

 phic agencies this material has been brought in contact with 

 highly heated carbonic acid and silica-bearing solutions, Avhich, 

 by removing the gi'eater part of the ealcic carbonate, and alter- 

 ing the magnesic carbonate to a silicate, have left the rocks in 

 the condition of hydrated magnesian silicates. During or at 

 the close of this period of metamorphism, the eastern edges of 

 the strata were tilted up, forming an anticlinal axis, while the 

 extension of the formation to the westward was subsequently 

 covered by the shale and sandstone deposited from the Triassic 

 sea. 



The true geological age of this belt of metamorphic rocks, 

 which ]'uns through Staten and New York Islands, extends far 

 northward through the New England States, where it has a 

 wide expansion, and has been traced sonthward as far as North 

 Carolina, is not definitely known. There have been three prin- 

 cipid theories advanced in regard to their antiquity ; these are — 



First. — That these rocks are of the same age as the Highlands 

 of New Jersey and the Adirondack Mountains, or of Lower 

 Laurentian age. 



Second. — 'J'hat they belong to the so-called Montalban sys- 

 tem, one of the several divisions of the Upper Laurentian distin- 

 guished by Dr. T. 8. Hunt and others. 



