REVIEWS 211 



beds and glacial drift later than it. The rocks had been folded and 

 ■eroded before the eruption began, but have been little disturbed since. 

 In general the igneous rocks represent the types resulting from the 

 differentiation of a granitic magma. The more abundant rocks are 

 highly siliceous and rich in alumina and the alkalies. They include 

 granite, granite-porphyry, quartz-porphyry, rhyolite, rhyolitic-obsidian, 

 rhyolitic tuffs and breccias. More basic rocks, including augite-diorite, 

 porphyries, passing into porphyrites, lamprophyric dikes and bosses 

 and basalt flows occur m less abundance. The rocks belong to five 

 groups : (i) The massive plutonic rocks represented by the main mass 

 which is a miraolitic granite becoming porphyritic at the edge, and a 

 second smaller mass, which is dioritic. The latter becomes a quartz- 

 diorite-porphynte at the edge, and is cut by aplitic dikes, probably 

 from the granite mass. (2) The porphyritic rocks of the intruded 

 sheets and flows include among the acid types micro-granite, quartz- 

 porphyry, granite-porphyry, feldspar-porphyry and porphyrites. The 

 basic types are lamphrophyric rocks, with phenocrysts of mica, augite, 

 hornblende and olivine. Dikes are, upon the whole, rather rare, and 

 in this particular the region stands in sharp contrast with the neighbor- 

 ing Crazy Mountain region. Minettes occur here as intruded masses 

 and sheets rather than in the usual form of dikes. (3) The extrusive 

 Tocks include rhyolites and breccias from the granitic mass, and basalts 

 in the region of Volcano Butte. (4) The tuffs and breccias have 

 yielded largely to erosion, as would be expected, and now make up 

 the Smith Lake beds. (5) There are certain igneous rocks in the region 

 which do not seem to belong to this center of eruption. These include 

 a diabase sheet intruded in the Belt shales (Algonkian), and presum- 

 ably very ancient, certain ash deposits in the Dakota, and certain dikes 

 of porphyrites, acmite-trachytes, trachytes and theralites of Crazy 

 Mountain types. 



The general order of eruption seems to have been : first, the dio- 

 rite (possibly not belonging to the main mass); second, the granite; 

 third, the rhyolite and pit.chstone ; and, fourth, the basalt and basic 

 dikes. It will be seen that the rocks became successively more highly 

 differentiated. The excellent analyses show that the alkali ratio K^O : 

 Na.^0 is about i : 1.55 — i : 1.30, with the most rapid variation at the 

 extremes. This ratio is independent of geologic position or coarse- 

 ness of crystallization, and seems to be characteristic of the magma. 

 A nunber of interesting petrographical facts are brought out in the 



