GEOLOGY OF THE BLUE MOUNTAIN QUADRANGLE 35 



granitic-looking and hornblende gneisses cut by masses of granitic 

 syenite and pink granite. 



The small body of mixed gneisses exposed along the river south- 

 southwest of Sprague pond shows not only normal syenite and 

 Grenville hornblende gneiss, but also a basic, igneous-looking gneiss 

 apparently an assimilation product. 



The other small areas mapped as mixed gneisses require no special 

 description. It is of course to be remembered that small exposures 

 showing the so-called " mixed gneiss " aspect are occasionally met 

 with in many parts of the quadrangle. 



Gabbro Stocks and Dikes 



Thirteen gabbro stocks or dikes have been discovered and sepa- 

 rately represented on the accompanying geologic map. They are 

 rather widely distributed over the quadrangle, though eight of them 

 are confined to its southern one-fourth. That these gabbro masses 

 are younger than any facies of the great intrusives already described 

 is clearly proved by their sharp contacts against those intrusives. In 

 many respects these gabbros are similar to those of the North Creek 

 quadrangle recently described somewhat in detail by the writer,^ 

 but they do not present so many variations. Most of them at 

 least have rounded to elliptical ground plans and practically vertical 

 contacts with the country rock, so that they are of the nature of 

 stocks, but in some cases where the exposures are not very satis- 

 factory, they may exist as dikelike forms. As a rule, the long 

 axes of the exposed gabbro bodies are almost, or quite, parallel to 

 the foliation of the country rock. No branching of any gabbro 

 mass was noted. The observed variations in size of the areas is 

 from a single small exposure a few rods across to others from 

 one-half to three-fourths of a mile long. 



A feature of particular interest is the almost invariable associa- 

 tion of pegmatite dikes or veins with the gabbros, the former cutting 

 the latter. The same thing has been noted in connection with the 

 gabbro stocks of the North Creek and Lake Pleasant quadrangles, 

 but its significance is scarcely known. 



Megascopically, the gabbros are seen to be medium to moderately 

 coarse . grained and dark gray when fresh with roughly equal 

 amounts of feldspar and dark minerals, the latter including horn- 

 blende or pyroxene or both, biotite, ilmenite and usually tiny reddish 

 garnets scattered through the rock. On weathering, the gabbros 



1 N. Y. State Miis. Bui. 170, p. 26-38. 1914. 



