﻿GEOLOGY OF THOUSAND ISLANDS REGION 33 



out such rocks either occur along granite-quartzite contacts, or 

 else are included in granite. They are apt to show close set, block 

 jointing, like the quartzite. They have been found only in asso- 

 ciation with the granite gneiss. The field evidence seems to us 

 strongly indicative of the fact that these are really intermediate 

 rocks, in the sense that they represent quartzites in various stages 

 of granitization ; that the quartzite is being permeated, soaked and 

 even digested by the granite. The character of the intermediate 

 rock, the shading of the two into one another, and the field oc- 

 currence of the intermediate stages, all point to this conclusion, 

 and seem incapable of explanation on any other hypothesis. 



Amphibolites. The name amphibolite is a convenient, compre- 

 hensive term for a group of rocks of gneissoid habit and dark color, 

 composed essentially of hornblende and feldspars, with often consid- 

 erable amounts of biotite or pyroxenes, and with accessory minerals 

 of which magnetite is easily chief, and quartz and garnet of fre- 

 quent occurrence. In respect to origin, the rock has long been a 

 puzzling one since apparently identically appearing amphibolite 

 might be produced by metamorphism from either igneous or from 

 sedimentary rocks of the proper character. In a multitude of locali- 

 ties in the Adirondacks it has been shown that gabbro intrusions 

 (whose character and origin is rendered certain by a core of prac- 

 tically unchanged rock) are largely changed over into amphibolites, 

 every step in the process being open to inspection. Similar rela- 

 tions have been shown in many localities in all continents. 

 Also in the Adirondacks, wherever the Grenville series is exposed, 

 bands of amphibolite of varying thickness are found so definitely 

 interstratified with other Grenville rocks of unquestioned sedimen- 

 tary nature, that there seemed no escape from the conclusion that 

 the rock must have resulted from the metamorphism of a sedi- 

 ment; and amphibolite of such origin is equally of world-wide dis- 

 tribution. In addition it has recently been shown by Adams that 

 amphibolite can also be produced on a large scale by the contact 

 action of granite on limestone. Here are therefore three different 

 modes of origin, and the rock may be either igneous, sedimentary, 

 or a contact rock. Each occurrence of the rock must therefore 

 be studied by itself, in so far as its origin is concerned. Amphib- 

 olite of all three types is present in our district. 



Within the mapped area amphibolite has not the bulk and im- 

 portance that it has in much of the Precambric district adjacent. 

 There is much of it present as inclusions in the granite gneiss 

 bathyliths and stocks, inclusions of much variation in size and in 



