FEATURES OF TIIK SOUTHERN AREA. 451 



The rock has a hypidiomorphous granular structure, is sometimes as 

 coarse as a coarse gabbro, but near the periphery becomes fine grained 

 and porphyritic and often lias a marked parallel structure due to motion 

 in the magma. A very basic coarse variety found near the center is 

 noticeable. The diorite is cut toward the center by masses of alight- 

 colored finer-graiiied granitite, which envelops fragments of the diorite. 

 It is surprising to see the similarity between this Tertiary diorite and 

 granite and the Paleozoic masses of similar rock found exposed on the 

 old eroded surfaces of the Atlantic states, as, for instance, on the northern 

 shore of Boston. In both cases the same black patches are seen in the 

 granite, referable here to enclosed dioritic fragments, and the same alter- 

 nations of basic and acid rock in streaks or " Schlieren " with parallel flow 

 structure. The diorite is found in place in the streams as low as the 

 8,000 feet contour and can be traced 2,000 feet higher, remaining quite 

 coarse. 



The Cretaceous shales dip gently off from the dioritic mass as a dome, 

 but at the actual contact were found sometimes turned up on ed»-e and 

 interlaminated with the diorite. The} r have been profoundly altered by 

 the intrusive rock, preserving in general the stratification of the thicker 

 hands hut losing all shaly structure. The result is a dense flinty handed 

 rock, creamy white, green, or black in color, resembling thecontact rock 

 called "adinole." or coarser, filled with biotite, and more like "hornfels," 

 a product of Paleozoic granite contacts. None of the zones of mineralogi- 

 cal change so characteristic of the latter were observed. This effect 

 extends out about 5,000 feet on all sides, gradually dying out, as certain 

 layers only are affected. 



The diorite stock as well as the adjacent Cretaceous rocks are cut by 

 later vertical dikes of diorite-porphyrite and allied rocks: these dike- 

 swarm in the contact zone, accompanied by horizontal and oblique sheets 

 of similar rock. Mr. -I. P. Iddings, who visited this place in 1891, find- 

 that the vertical dikes, both in the stock and in all this part of the range, 

 have a general radial arrangement, with the diorite mass as an approxi- 

 mate center, repeating a fact observed by him in a smaller diorite stock 

 in the Yellowstone mountains. These Ion-- radial dikes extend outward 

 even into the benches at the southern base of the range. 



This imperfed description can give hut a fainl idea of the beauty of 

 this -Teat massiv and its eontad ring. Its intrusion into nearly horizontal 

 late Cretaceous strata and the enormous subsequent erosion which re- 

 moved the overlying rocks enable us to see it now in nearly its original 

 condition with deep sections into its center, whereas in the older inn--. - 

 of granitic rock which we usually study the long-continued erosion and 



