Granite at Mounts Adam and Eve. 641 



north from Edenville and then northwest in a general way along 

 the line of section 3 (see Map and Sections, Plates II and III), within 

 about 200 feet of each other, and on the northeast slopes of Mt. 

 Eve, and at the north end the two are much mixed up together. 

 It is doubtful if a very pronounced contact w^ould show at any point, 

 even were the soil all stripped away. The blue becomes graphitic, 

 and notably crystalline towards the white, and in a series of speci- 

 mens almost all intermediate grades may be illustrated. Where, as 

 on the eastern side of Round Hill, the blue approaches closely to 

 the granite, it is excessively siliceous, if not actually a quartzite, 

 and hence not a favorable material for contact effects. On the whole 

 we felt forced to the opinion that the white is metamorphosed blue, 

 and we think it most reasonable to attribute the change to the 

 granite intrusions. The petrographic details and local observa- 

 tions will be next given. 



Petrography of the Granitic Rocks of Mt. Adam and Mt. Eve. 



On the eastern side of Mt. Adam and the western side of Mt. 

 Eve quite extensive quarries have been opened, where they are 

 indicated by the conventional sign on the map. Practically the 

 same rock as shown by the slides makes up both ridges, although, 

 in places, it is fairly gneissic. It is a hornblende granite, rather 

 coarsely crystalline at the quarries, with its individual components 

 appearing a J to J inch in diameter. It resembles the w^ell-known 

 Quincy granite somewhat, but is rather darker and has a pinkish 

 cast in with the green, due to the varying tints of the feldspar. 

 More or less biotite is also present, and, especially near the edges, 

 some augite. Under the microscope these larger crystals break up 

 into smaller ones, and the actual size is nearer ^--\ inch. 



The granite is at once seen to be a very basic variety, with quite 

 as much plagioclase as orthoclase, and in instances even more. The 

 rock might with almost as much propriety be called a quartz-diorite. 

 It suggests strongly the " diorite-granites" of S. F. Emmons (Ge- 

 ology of Butte, Mont., Trans. Inst. Min. Eng., xvi, 52, and Geologi- 

 cal Sketches by same author), which are so important in Butte, and 

 elsewhere in Montana, where the same variability occurs. Quartz 

 is quite abundant, but not especially so. It is charged with the 

 usual dusty inclusions, solid, liquid, and gaseous. Normal ortho- 

 clase is not especially common, but is largely replaced by microcline 



