GEOLOGY OF LAKE PLEASANT QUADRANGLE 2^ 



On the prominent mountain spur 2 miles east of Whitehouse, the 

 rock toward the base is mostly normal syenite which, about half 

 way up the mountain, gives way to a prevailing granitic syenite, 

 but with all types from hornblende syenite to pink granite or granite 

 porphyry grading into each other rapidly. 



Along the creek from one-half to i>4 miles north of Whitehouse, 

 there are good ledges of syenite to granitic syenite, chiefly the latter, 

 which are often pink and variable in appearance because of occa- 

 sional dark gneiss streaks or bands. This rock strongly suggests a 

 transition one from ordinary granitic syenite to the pink granite 

 with its amphibolite bands below described as occurring on Hamil- 

 ton lake stream. 



These detailed descriptions are here given to show that, so far 

 as the writer's observations are concerned, there is no evidence for 

 different ages of normal syenite, granitic syenite, and pink granite, 

 but rather there is much evidence that these types grade into each 

 other and are really only different phases of the same great plutonic 

 body. In no case has one of these rocks been found definitely to 

 cut another, though rapid variations and gradations are frequent 

 and many times these closely involved rocks are intimately associated 

 with streaks of Grenville gneiss. Such Grenville shows all stages 

 of having been more or less assimilated or incorporated by fusion 

 into the igneous masses. 



Thus it is difficult to resist the suggestion that some at least of 

 these acidic variations of the normal syenite are to be accounted for 

 on the basis of magmatic assimilation where acidic Grenville has 

 changed the composition of the syenite magma. This view does 

 not, however, preclude the possibility of considering other acidic 

 variations from the normal syenite as due to a process of pure 

 differentiation as is probably true of the Mount Francisco mass 

 above described. The writer inclines to the belief that such pure 

 differentiation has been the principal factor in the production of 

 the large masses of granitic syenite and granite. Still again, the 

 more or less perfect assimilation of Grenville gneiss fragments by 

 the granitic portion of the magma has quite certainly caused many 

 of the puzzling, rapid, local variations of the rock by the formation 

 of masses of intermediate character. 



Granite 



Approximately 35 square miles of the area of the quadrangle are 

 occupied by granite. This is a reddish to grayish granite gneiss 

 which, for most part at least, is quite certainly only the extreme 



