38 



NEW YORK STATE MUSEUM 



(below described) in two important respects, namely, that it was 

 practically confined to a single great area of 1200 square miles, 

 and that it broke through the Grenville only, no other country rock 

 having been present at the time of the intrusion. A few small 

 masses of anorthosite are known to occur outside of the great area 

 as, for example, in the central part of the Long Lake quadrangle; 

 northern part of the Blue Mountain quadrangle; northeastern part 

 of the Lake Pleasant quadrangle ; and near Rand Hill in Clinton 

 county. For most part the molten anorthosite appears to have 

 pushed aside or displaced the Grenville, though in many cases small 

 areas and fragments are seen enveloped by the intrusive, while in 

 a few cases there appears to have been injection or possibly assimi- 

 lation of the Grenville. That this anorthosite is younger than the 

 Grenville is demonstrated by the facts that tongues of the rock have 

 been observed cutting through the Grenville (see figure 6), and 

 also that fragments of the old strata are frequently seen to have 

 been torn off and completely enveloped in the molten anorthosite. 



E3 



GrenuiJle 

 series 



^, 



■orthosiie 



1^ i' 



lP\Sijenite -ff ramie 1+ .-t-+| Gabbro or 

 nA .series \±_±!j^diobase dj/ie 



Fig. 6 Generalized section showing how the relative ages of the most 

 common Adirondack rocks are determined. Where one rock mass cuts 

 through the other, the former is the A'ounger. (By W. J. Al.) 



The next clearly recorded event after the intrusion of the anor- 

 thosite was very widespread igneous activity when the rock known 

 as the syenite- granite series, so common in the Adirondacks, was 

 forced upward in a molten condition into both the Grenville and 

 the anorthosite. There may have been more than one time of 

 intrusion, as argued by certain workers for the St Lawrence valley, 

 but, for our purpose, it v/ill suffice to regard all the syenite-granite 

 rocks to have been intruded at practically the same time, since the 

 sum total of effects is much the same as though there had been 

 only a single period of igneous activity. All portions of the 

 Adirondacks felt the force of the intrusion of these molten masses 

 which are now represented by numerous large and small areas very 

 irregularly scattered throughout the region. Almost all the higher 

 mountains outside of the anorthosite area consist of syenite or 



