GEOLOGY OF LAKE PLEASANT QUADRANGLE 1/ 



Basic phases of the syenite. As above stated, the normal syenite 

 in one direction grades into rocks of basic (dioritic or even gabbroic) 

 composition. Only five areas of such rocks are of sufficient size 

 to be represented upon the geologic map, and altogether these are 

 only about 8 square miles in extent. There are, however, in other 

 portions of the quadrangle, especially in the mixed gneiss areas, 

 some small rock masses of the same general character that have 

 quite certainly been produced by magmatic assimilation of Grenville 

 by granite or syenite. 



Mineralogically the basic syenite differs from the normal syenite 

 chiefly in its usual absence of quartz, microperthite, and orthoclase; 

 predominance of plagioclase (often basic) ; and the frequent pres- 

 ence of scattering, small to large clear-red garnets. Many times 

 rocks of transitional character occur like those represented by slides 

 number 9 and 15 in the table on page 16 and at various places in 

 the field. Often, as on the eastern face of Speculator mountain and 

 on the mountain side a mile southeast of Sand lake, these basic 

 rocks have a decided gabbroic appearance so that in certain cases 

 the distinction between these and the latter gabbros can not always 

 be made with certainty, as will be pointed out in the discussion of 

 these later gabbros. 



As regards color, texture, and structural features, the same sorts 

 of variations described in connection with the normal syenite apply 

 here. 



Some years ago, in the Diana-Pitcairn area of the western Adiron- 

 dacks, Smyth ^ proved that the prevailing normal augite syenite at 

 times shows a perfect transition to a distinctly more basic gabbroic. 

 rock. Judging from his description, the phenomena there are much 

 like those here considered. 



Within the Long Lake quadrangle Gushing^ has found a basic 

 phase of the normal syenite, but it always lies along the border 

 between syenite and anorthosite and hence strongly suggests the 

 incorporation of some of the anorthosite into the molten syenite to 

 produce the basic phase. 



Recently the writer has proved ^ that, on Gore mountain in War- 

 ren county, a long narrow zone of basic syenite has been produced 

 by melting and assimilation of the border of a large inclusion of 



1 Geol. Soc. Am. Bui. 6, 1895, p. 271. 17th Annual Rep't N. Y. State Geol., 

 1897, p. 472-74. 



2N. Y. State Mus. Bui. 115, p. 479. 



3 Econ. Geol., vol. 7, no. 5, 1912, p. 498-501. Also N. Y. State Mus. Bui. 

 164, p. 100-2. 



