﻿34 NEW YORK STATE MUSEUM 



abundance. Frequent bands of it occur within the Grenville series, 

 but these are usually of no great thickness. There is but little of 

 the rock present to which an igneous origin may be definitely as- 

 signed. There are small areas of such rock in the district north and 

 northeast of Theresa, where a somewhat more heavily bedded am- 

 phibolite occurs, which holds much pyroxene in addition to the 

 hornblende, and which seems to definitely cut the limestone with 

 which it is associated. There are, however, amphibolite bands in- 

 terstratified with the same limestone, and the mass has been severely 

 deformed, with the production of flow in the limestone and the 

 fracturing of the amphibolite into blocks, making one appear to 

 cut and be included in the other, but this does not seem to be a 

 case of the sort. In our experience amphibolites which result from 

 the metamorphism of gabbro, usually contain pyroxene in quantity, 

 while those originating from calcareous shales are more apt to be 

 micaceous and lack the pyroxene, but this is far from being an in- 

 variable rule, and is only suggestive of origin, not demonstrative. 



The amphibolites interstratified within the Grenville series, and 

 regarded as metamorphosed sediments,, calcareous shales or some- 

 thing of that sort, are mostly quite finely and evenly granular rocks, 

 which have wholly recrystallized, and vary from very solid look- 

 ing, dense rocks in which mica is but sparingly present, to very 

 schistose, highly micaceous rocks, which rapidly break down under 

 the weather. In most of these orthoclase feldspar is apt to predomi- 

 nate over plagioclase, and much of the rock contains some quartz, 

 the micaceous varieties often considerable. The manner in which 

 •the variations appear is itself highly suggestive of metamorphosed 

 sediments which differed somewhat in character from bed to bed. 

 Some of the rock contains garnets, in some cases reaching large 

 size, but they are exceptional rather than the rule. 



The amphibolite of contact origin will be discussed under the 

 general topic of contact rocks. 



Schists. Under this heading are included a large number of 

 rock types, so many that it seems hopeless to attempt to describe 

 all, or many of them. No doubt they have diverse origins. Some 

 of them quite certainly owe their present character to contact action, 

 and no doubt contact action of varying%ind, and in varying degree, 

 is in large measure responsible for the great diversity of the group. 

 Some of the rocks grouped here are no doubt igneous, and in their 

 character distinctly suggest such an origin, though the proof is dif- 

 ficult to obtain. 



A very common variety of Grenville schist, the so called w rusty 



