﻿GEOLOGY OF THOUSAND ISLANDS REGION 53 



is of light brown color, in the coarser varieties very light brown, 

 resembling muscovite, though it seems undoubted phlogopite. 1 

 Scapolite is also an abundant mineral in these zones, a phlogopite- 

 scapolite-calcite rock being the usual combination. This is not 

 one of the customary types of Grenville contact rocks in the general 

 region, though the common one here. 



There are two other types of contact rocks which occur in small 

 quantity within the area here, though common enough elsewhere, 

 which call for brief attention. They occur in the district east of 

 Redwood where Grenville rocks of all types are cut by small gran- 

 ite masses. One is a heavy, basic, black rock, weathering rapidly, 

 and composed chiefly of green pyroxene and black hornblende, with 

 a little graphite, considerable pyrite, and some 15$ of calcite re- 

 maining. Heavy, pyroxenic rocks of this type occur throughout 

 the Adirondack region at limestone contacts, though usually not 

 so hornblendic as this rock. 



The other rock consists of large, gray green pyroxenes set in a 

 felt of tremolite needles, with rather abundant pyrite as the only 

 accessory mineral. Such tremolite rocks occur not infrequently in 

 the Grenville, the tremolite quite commonly altering to talc. The 

 especial interest attaching to this particular exposure is that the 

 tremolite rock is developed at the contact of granite against Gren- 

 ville rusty gneiss, and seems quite certainly a result of the con- 

 tact action of the one upon the other. So far as we recall, just that 

 type of contact action has not heretofore been noted in the region. 



Great Precambric erosion 



The Grenville rocks are the only Precambric sediments in the 

 region, and are of very early Precambric age. The remaining rocks 

 of this age in the district are all igneous, and there is no evidence 

 that any later Precambric sediments were ever deposited here- 

 abouts, though it is possible that some such were deposited and 

 subsequently worn away. The Precambric rocks of the present 

 surface, both sedimentary and igneous, present characters which, 

 so far as we know, are only given to rocks under conditions of high 

 pressure, and at least moderately high temperature, conditions which 

 in general prevail only at considerable depths below the surface. 

 All the igneous rocks except the diabases give evidence that they 

 solidified well beneath the surface, and the deformation of both 

 these and the sediments is of deep-seated type. It is, however, not 



1 It is of the second order and with very small axial angle. 



