GEOLOGY OF LAKE PLEASANT QUADRANGLE II 



be clearly understood that this Grenville area, like most of the 

 others, is not always sharply separated from the inclosing rocks, 

 there frequently being so-called mixed gneisses along the borders. 



The nine lenslike inclusions of Grenville shown in the southern 

 one-third of the quadrangle have strikes parallel and rocks similar 

 to those of the large area just described. These inclusions are 

 certainly only smaller fragments of the Grenville caught up in 

 the invading magma. Besides these, many small unmappable 

 inclusions were found and doubtless others were undiscovered in 

 this very rough country. 



In the second largest Grenville area, which occupies a little over 

 2 square miles northeast of Wells, outcrops are scarce except in its 

 northern portion. Along each river, about one-fourth to one-half 

 of a mile above their confluence, are big ledges of Grenville white 

 gneiss which in thin-section shows the following mineral percent- 

 ages : feldspars (orthoclase, microcline, microperthite and oligo- 

 clase) 60; quartz 37; small, clear-red garnets 2; and muscovite i. 

 The limited exposures in the southern half of the area indicate the 

 prevalence of a light-gray feldspar gneiss much like that just de- 

 scribed but with no garnets and with tiny mica flakes making up 

 several per cent of the rock. Judging by its general appearance, 

 banded structure, variableness, and close association with limestone 

 in at least one place, this rock is quite certainly Grenville. This 

 white gneiss, as well as certain others of the quadrangle, is almost 

 identical in composition with that very recently regarded as of 

 igneous origin by Gushing ^ in the Saratoga Springs region and by 

 him somewhat doubtfully called " Laurentian " granite. The field 

 relations and character of the white gneiss of the Lake Pleasant 

 region do not favor its igneous origin. No pegmatitic phases of the 

 white gneiss were there noted. Furthermore, as is pointed out in 

 the discussion of the granites, there are considerable areas of reddish 

 granites within the quadrangle whose color, field relations (includ- 

 ing long, narrow, amphibolite inclusions), etc. are almost precisely 

 like the so-called '' Laurentian " granite of the Thousand Islands 

 district, but the writer does not believe that both these red granites 

 and the white gneisses can possibly be of the same age and origin 

 within the Lake Pleasant quadrangle. 



Four outcrops of limestone occur in the white gneiss area just 

 described. The largest, about 100 feet long, is associated with the 

 white gneiss and lies in the stream bed one-fourth of a mile above 



1 N. Y. State Mus. Bui. 169, 1914, p. 21-25. 



