260 SILURIAN ROCKS. 



Our chief objection to this classification is the use of the term epoch. An epoch is the 

 point of time when an event takes place, not the interval between two points of time, 

 which is properly a period. 



In speaking generally of these different formations, we may say that the Potsdam sand- 

 stone is a purely silicious sandstone; the calciferous sandrock is intermediate in character, 

 as it is also in position, between purely silicious and purely calcareous beds ; the chazy 

 limestone, birdseye limestone, Black River limestone and the Trenton limestone are toler- 

 ably pure limestones ; the Utica slate is a slaty limestone or calcareous slate, forming a 

 transition between the limestones below, and clay slates above. The Hudson River group 

 is chiefly clay slate, but includes also much calcareous slate and thick beds of limestone 

 in its upper portion. 



The Potsdam sandstone rests unconformably upon crystalline rocks invariably. In 

 general the higher formations of the lower silurian scries rest conformably upon the Pots- 

 dam sandstone, and upon one another. There are generally a few degrees of difference in 

 the dip between the Hudson River group in Vermont and the overlying red sandrock. 

 Many geologists do not think this difference sufficiently great to be ranked as a constant 

 unconformability. 



SNAKE MOUNTAIN SECTION. 



Upon none of the lai'ge sections appended to the Report is there a complete view of the 

 lower silurian rocks. All of them, however, except the first five, embrace some of them. 

 There is no place in Vermont where the whole may be found in direct succession. They 

 are all exhibited, except the Potsdam sandstone, upon Snake Mountain in Addison. To 

 illustrate this group of rocks we will give a detailed account of this section. For sections 

 illustrating the position of the Potsdam sandstone we refer to figures under the description 

 of that rock, which show its relations to the Laurentian gneiss below, and the calciferous 

 sandrock above. 



Snake Mountain is one of the most noted localities in Vermont, as respects Geology. 

 It was first described by Prof. Emmons in his Report upon the Geology of the Second 

 District of New York, pages 279 282. This account is in accordance with our views. 



Prof. Adams gave a section of the rocks of this mountain in his Second Annual Report 

 upon the Geology of Vermont, pages 162, 163. We find little that is essential to add to 

 his account, except to draw it out more fully. 



Prof. Emmons having become convinced that there is a system of rocks containing 

 fossils below the lower silurian (which he has given the name of Taconic), re-examined 

 Snake Mountain subsequently to the publication of his Report, and came to the conclusion 

 that in addition to the calciferous sandrock, chazy limestone and Trenton limestone, at 

 the base of the mountain, the black slate of the Taconic System was present (what was 

 before called Hudson River slates), underlying the calciferous sandrock; the rock which 

 he had previously called the equivalent of the (/ray sandstone. These new views are 

 brought out in the first volume of his work on North American Geology, Vol. I, Part II, 

 pages 87, 88 and 89. 



