GEOLOGY OF THE BLUE MOUNTAIN QUADRANGLE 



25 



Basic (dioritic to gabbroic) facies of the syenite. The rocks 

 inchided under this caption really have the composition of diorite 

 or gabbro though they have been quite certainly produced either by 

 pure differentiation of the syenite magma or by the assimilation 

 of country rock through which the syenite magma was intruded. 

 They are quite dift'erent in aspect from the typical later gabbros 

 which are separately represented on the geologic map in that they 

 show neither the diabasic texture nor the peculiar mottled appear- 

 ance of these later gabbros. In fact the color, texture and structural 

 features are essentially the same as those of the normal quartz 

 syenite already described. 



Mineralogically, the basic facies of the syenite differ from the 

 normal syenite chiefly by absence of microperthite, reduced amount 

 or absence of orthoclase and quartz, and predominance of plagio- 

 clase (oligoclase to andesine). 



The following tabular summary illustrates the mineralogical com- 

 position of typical thin sections from several areas of the so-called 

 basic syenite. 



Table 4 — Thin-sections of basic (dioritic to gabbroic) phases of the 



syenite 







^ 



^ 



-d 



c 









^ 













6 

 c 







1 







1 



2 



1 



1 



Q 



.2 

 m 







1 



1 

 



>> 



Oh 



c5 



1 



■J 



2 . . . . 



1 

 ih 6 23 



Ol.-An. 35 







15 



10 



I 



15 





^ 







36 



12b i' 



Ol.-An. 33 



25 



T2 



8 





8 



T 



12 





little 



little 



44. . . . 



iSb 3 8 



Ol.-An. 26 



35 



20 



5 







4 





T^ 



little 



little 





ss 



i7f 9 



Ol.-An. 70 



I 



2S 









3 





I 



little 



little 



little 



S6 



I7e 4 . . . . 



Ol.-An. 42 



35; 







15 



I^ 



6 





Uttle 



little 



little 















No. 2, 2y2 miles south-southeast of Sprague pond; no. 36, Buttermilk falls 

 on Raquette river; no. 44, eastern base of Owl's Head mountain; no. 55, 

 eastern shore of Long lake i^ miles northeast of the Long Lake bridge; 

 no. 56, eastern shore of Long lake 1% miles northeast of the Long Lake 

 bridge. 



The rock in the small area near the middle southern border of 

 the quadrangle (no. 2, table 4) is pale greenish gray, medium 

 grained, clearly gneissoid, with occasional crystals of biotite and red 

 garnet set in a granular mass of feldspar. This rock appears to 

 grade into the surrounding normal syenite. 



In the area including Buttermilk falls there are many outcrops, 

 the best being at the falls where, in a great ledge, the rock (no. 36, 

 table 4) is mostly rather homogeneous, greenish gray when fresh, 



