244 THE CAMBRIAN. Ibull.81. 



POTSDAM. 



There are several early references to a sandstone occurring at the 

 base of the section in the Upper Mississippi Valley and about the Adi- 

 rondack Mountains in New York ; but it was not until 1838 that it was 

 well defined and given a distinct name. This was done by Dr. E. Em- 

 mons. In describing this sandstone, Dr. Emmons says: 1 



I shall not enter upon its geological relations any farther than, to state (hat, in 

 Potsdam and other towns in which it appears, it uniformly rests on the primary 

 strata ; and in no part of the county is there any rock which interposes itself between 

 it and the primary, so that ii appears here as the oldest representative of the transi- 

 tion series. The identification of this rock with the sandstones along the southern 

 border of Lake Ontario will be a matter of some difficulty. It is geologically below 

 the transition limestone, and never in the northern district alternates with it, but 

 always holds the relation of an inferior rock. * * * 



This rock is a true sandstone, of a red, yellowish red, gray, and grayish white colors. 

 It is made up of grains of sand, and held together without a cement. Intermixed with 

 the siliceous grains are finer particles of yellowish feldspar, which do not essentially 

 change the character of the sandstone, but they show the probable source from which 

 the materials forming it were originally derived, viz, some of the varieties of granite # 



On the Lake Champlain side of the Adiron&acks it is described as 

 follows : 



Its position is evidently beneath the transition limestone and calciferous sandrock. 

 It is very deficient in organic relics, though not entirely destitute of them. It is 

 unnecessary to repeat what has already been said of this rock; it is purely quartzose 

 or siliceous in its composition and finely stratified. It dips to the northeast at Port 

 Kent at an angle varying from 5° to 10°. The places where it occurs along Lake 

 Champlain are indicated on a map of apart of this [Clinton County], to which I refer 

 the reader. This rock at Keeseville has been rent in the most remarkable manner; 

 several fissures, the principal one of which extends nearly a mile, and through which 

 the Au Sable flows, have been opened by some convulsion in nature to the depth, in 

 some places, of a hundred feet and from five to twenty wide. Near the bottom of the 

 fissure at High Bridge, as it is called, I discovered numerous specimens of a small 

 bivalve molusca, a lingula. I found, also, on examination, that the same fossil 

 occurred through an extent of 70 feet at least, and so far as 1 could discover, it was 

 the only fossil inclosed in the rock. It is extremely thin and delicate, yet the shell 

 is perfectly preserved, and is probably one of the oldest inhabitants of the globe, as 

 the rock in which they occur is the oldest of the transition series. 2 



The name "Potsdam sandstone" was quickly accepted by the New 

 York State geologists, incorporated in their annual reports, and used in 

 the final report on the geology of the State. The extension, by corre- 

 lation, of the name to the Southern Appalachians, through Pennsyl- 

 vania, Virginia, North Carolina, Tennessee, and Alabama ; westward 

 through the Mississippi Valley and the Kocky Mountains, and north 

 into Canada, and down the St. Lawrence Valley, rapidly followed, until 

 the name Potsdam is now used to typify the Upper Cambrian zone in 

 all parts of America. 



1 Report of the geologist of the 2d geo'ogical district of New York. Second annual report of the Qeo- 

 '.ogiral Surrey of N. Y. Albany, 1838, pp. 214. 215, 

 'Op. cit.,p.230, 



