462 THE AZOIC SYSTEM AND ITS SUBDIVISIONS. 



metamorphic rocks, are of Lower Silurian age ; also, that the limestone 

 and the conformably associated rocks of the Green Mountain region, 

 from Vei'mont to New York Island, are of the same age. He remarks 

 furthermore : — 



" These Westchester County rocks have been pronounced Montalhan. I 

 know of no facts sustaining such a conclusion. If true, it would follow, from 

 the above, that the original Montulban rocks — those of the White Moun- 

 tains — also are Lower Silurian." (Am. Jour. Sci., 1880, (3) XX., p. 455.) 



Of the correctness of these statements in regard to the Lower Silu- 

 rian age of the rocks in question, it seems to us that there can be no 

 possible doubt. 



NEW YORK. 



The mass of the Adirondack Mountains is composed, in large part, of 

 highly crystalline rocks, among which that called " hypersthene rock " 

 by Professor Emmons is the most conspicuous and important, forming 

 as it does the highest portion of the chain, and covering, according to 

 the same authority, the whole of Essex County. Gneiss, granite, and 

 syenite are also present over areas of some extent, while mica schist 

 and talc schist are said to be entirely wanting. The extent and impor- 

 tance of the masses or beds of magnetic and specular oxides of iron are 

 also well known. In Professor Emmons's classification of the Adirondack 

 formation, gneiss and syenite are put down as being stratified, while the 

 limestone is considered as being unquestionably of eruptive origin. 



The Adirondack area is admitted by geologists — without exception, 

 it is believed — to be Azoic, or ArchiTean. In view of what has been 

 stated in the preceding pages with regard to the facility with which the 

 Azoic rocks of other regions in this county have been assigned to be 

 Laurentian, Huron ian, or Montalban, or moved backwards and forwards 

 between these three supposed systems to suit the fancy of the theorist, 

 it seems rather remarkable that the rocks of Northern New York sliould 

 have been so little meddled with in this way. Our knowledge of the 

 geology of the Adirondack region is so exceedingly incomplete, that a 

 fine field lies ready there for theoretical speculations of a kind simi- 

 lar to those which we have shown to have been so prevalent in regard 

 to the rocks of New England. A brief statement will here be given of 



