354 NEW YORK STATE MUSEUM 



The syenite porphyries are somewhat lower in lime and higher 

 in alkali percentage than the corresponding syenites. But the 

 differences are not thought to be sufficiently large to condemn a 

 reference to the same parent magma. 



Paleozoic rocks 



Potsdam (Cambric) sandstone. Ljn^ng uncomformably on the 

 old and much eroded Precambric surface, a great sandstone 

 formation appears, on the north and east and on the eastern half 

 of the southern border of the Adirondack region. This is a water- 

 deposited formation, and, so far at least as its upper portion is 

 concerned, a marine formation. It is thickest on the northeast, 

 thinning out to disappearance both to the south and west. As, 

 furthermore, it appears to be the upper beds which persist, and 

 the lower ones which disappear in these directions, it seems cer- 

 tain that, so far as the immediate region is concerned, the marine 

 invasion came on it from the northeast and extended progressively 

 southward and westward. 



In Clinton county, where the formation is thickest, the basal 

 portion is rather sharply differentiated from the rest in character, 

 and this portion has considerable thickness, though how much, 

 and how large a part of the whole thickness it constitutes, is 

 wholly uncertain. The writer was the first to show this, and 

 it has lately been reaffirmed hj van Ingen.^ This portion con- 

 sists in part of coarse basal conglomerates, in part of poorly in- 

 durated sand beds of small durability and in smaller part of 

 thoroughly indurated sandstones. It is nearly everywhere char- 

 acterized by a considerable feldspar content, in addition to the 

 quartz, and this feldspar is for the most part fresh. Con- 

 siderable magnetite also appears in places, along with grains of 

 garnet and occasional zircons. The rock has therefore an arkose 

 character in this portion, while above it is prevailingly of pure 

 quartz sand. Red is the predominant color of the base, and there 

 is but little white sandstone in it, while above the latter is the 

 prevailing color. As a general proposition, a feldspar content 

 and a prevalence of red beds go together and are certain signs 

 of the basal portion. 



^N. Y. state Mus. Bui. 52, p.543. 



