GEOLOGY OF THE SCHROON LAKE QUADRANGLE 7 



GENERAL GEOLOGIC FEATURES AND PUBLICATIONS 

 Most of the more common and well-known rock formations of 

 the Adirondack region, as well as certain unusual ones, are abund- 

 antly represented in the Schroon Lake quadrangle. In the regular 

 order of their geologic ages the principal rocks are as follows : 

 Pleistocene 



Glacial and Postglacial deposits 

 Paleozoic (Cambrian) 



Little Falls (?) dolomite 

 Potsdam sandstone 

 Precambrian 

 Diabase dikes 

 Aplite and pegmatite dikes 

 Gabbro stocks and dikes 

 Keene gneiss, and assimilation product of the anorthosite and 



syenite-granite series 

 Syenite-granite series 

 Anorthosite series 

 Grenville series of metamorphosed strata ■ 



"The Precambrian formations constitute the foundation rocks of 

 the entire quadrangle. Oldest of all are the Grenville strata, prob- 

 ably of Archeozoic age, which are thoroughly crystalline. Gren- 

 ville strata are well developed in the southern half of the quad- 

 rangle, but their distribution is very " patchy '' because they were 

 invaded and cut to pieces by great masses of intrusive rocks. 



Next to the oldest known is the anorthosite series which occupies 

 most of the northeastern half of the area. The great bulk of this 

 rock is very coarse grained and highly feldspathic (Marcy anor- 

 thosite), but it has a more or less well-defined border development 

 (Whiteface anorthosite). 



The syenite-granite series, so common throughout the Adiron- 

 dack region, is next in order of age, being clearly intrusive into 

 both the Grenville and the anorthosite. This series, more particu- 

 larly its granite facies, is w^holly confined to the southwestern half 

 of the quadrangle where it is the most prominent rock formation. 



Several considerable areas of Keene gneiss lie in general between 

 the anorthosite and syenite or granite areas. This rock is regarded 

 by the writer as an assimilation- product which resulted from the 

 incorporation and digestion of anorthosite by the molten syenite- 

 granite. 



