510 



Report of the State Geologist. 



body. When not too finely granulated these basic gabbros show a tendency 

 to the assumption of ophitic structure. Such consist of coarse plagioclase 

 and nearly colorless pyroxene, both extraordinarily full of minute inclusions, 

 embedded in a granular matrix, in large part constituted of secondary min- 

 erals, garnet, biotite, hornblende and hypersthene with or without quartz. 

 The feldspar and pyroxene were apparently crystallizing at the same time, 

 for, while the feldspar is frequently idiomorphic against the pyroxene, it also 

 frequently includes the latter, while the reverse has not been observed to 

 occur. These varieties commonly show beautiful reaction rims around the 

 magnetites, similar in all respects to those described by Professor Kemp in 

 the Port Henry gabbro. 



Series IV. Unconformably overlying the older crystalline rocks is a 

 group of great thickness of nearly unchanged sedimentary rocks of Cambrian 

 and Ordovieian age, which hides the older rocks from view over two-thirds of 

 the county. Unfortunately exposures are not always all that could be 

 desired, and in regard to a small part of the group the stratigraphy is not 

 clear. The whole is seriously in need of thorough palaeontologic study, which 

 could not fail to furnish results of great interest and value. The group com- 

 mences at the bottom with the Potsdam sandstone, and terminates with the 

 Utxca slate. No criteria have been found, by means of which an older 

 quartzite series can be discriminated from the Potsdam, as urged by N. H. 

 Winchell for northern New York.* The quartzites and quartz schists of the 

 Grenville series of Canada seem wholly or largely lacking in the eastern 

 Adirondacks, though they may occur on the west. Furthermore, they are an 

 integral part of the series. In the absence of fossil evidence, no division of 

 the Potsdam seems possible in Clinton county, though it is far from certain 

 that the whole is of Upper Cambrian age. 



It is by no means impossible that rocks representing the Lorraine stage 

 of the Hudson river group have been deposited on the west side of 

 lake Champlain, but nothing younger than the Utica slate has yet been 

 discriminated. 



Potsdam sandstone. The Potsdam is widely exposed in the county, 

 forming the rock surface in two townships and a large part of it in three 

 others. Out of the fourteen towns making up the county, there is but a 

 single one (Black Brook), in which this formation is wholly wanting. It is 

 also found at very varying altitudes. At Ooopersville and in the Ausable 

 chasm it is close to the level of the lake, and at the former place the horizon 



* American Geologist, October, 1896, p. 207, and Twenty-first Annual Report of Geological Survey of Minn., pp. 99 112. 



