﻿IO NEW YORK STATE MUSEUM 



the more basic rocks which appear in large quantity further east 

 are but sparingly present. But granitic intrusion took place on a 

 large scale at least twice, probably three times, and possibly sev- 

 eral times. This it was which was so effective in breaking up, al- 

 tering and destroying wholesale the Grenville sediments and their 

 floor. 



Laurentian granite gneiss. The oldest of these igneous rocks 

 is a granite which has, since its intrusion, been sufficiently sub- 

 jected to compression to have become pretty thoroughly crushed, 

 or granulated, with the development of a rude foliated, or gneis- 

 soid, structure. It is a reddish to gray granite gneiss which con- 

 tains nearly everywhere inclusions of the Grenville rocks in vary- 

 ing abundance, but always most abundant near the contacts with 

 the Grenville, into which it always sends a multitude of dikes. 

 The inclusions are usually of amphibolite and all stages of their 

 assimilation by the granite are found, giving rise to a group of 

 intermediate rocks which seem unquestionably to have been de- 

 rived from the digestion of the one rock by the other. It is pos- 

 sible that some of these amphibolite inclusions may actually repre- 

 sent fragments of the old Grenville floor, and furnish the sole re- 

 maining traces of that floor, but as yet this is mere conjecture. 

 This granite gneiss occurs in both large and small masses, so called 

 bathyliths and stocks, which invaded the Grenville rocks from be- 

 neath at an exceedingly early period. 1 In addition to forming a 

 large portion of the present surface occupied by the Precambric 

 rocks it likely also underlies the Grenville rocks over the entire 

 district, except where they have been cut away by succeeding 

 igneous rocks. Since the rock solidified it has been subjected to 

 compression, together with the Grenville rocks, giving to each a 

 foliation parallel to that of the other, and elongating the bathyliths 

 in a northeast-southwest direction with corresponding shortening 

 at right angles to this, the shortening being of course in the di- 

 rection of the pressure and the elongation at right angles to it. 



Alexandria syenite. On the Alexandria quadrangle, some 3 

 miles a little west of north of Redwood, is a mass of rather coarse 

 grained igneous rock which shows little sign of crushing and is un- 

 questionably younger than the Laurentian granite gneiss. In as- 

 sociation with it is a much greater amount of a coarse, but crushed, 

 porphyritic igneous rock, now converted into an " augen " gneiss. 



1 Bathylith is a term applied to large masses of igneous rock, which masses 

 are believed to continue to great depths with generally increasing size 

 downward. A stock is a smaller mass of the sort. 



