674 THE GEOLOGY OF MINNESOTA. 



[Gabbro. Basalt. 



No. 1200. GABBRO. 



Near Otter Tail P. O. (original Huronian region), Canada. 

 Ref. Annual Report, xvi, page 29. 



Meg. Gray, medium grained, indefinite. 



Mic. The rock has been much broken by dynamic movements, and the altera- 

 tion of all the minerals is considerable. Particularly has zoisite been developed, 

 indicated by its high refractive index, its parallel extinction and its blue color between 

 the nicols. Much pyroxene is evident, but no olivine, while the feldspar is unidenti- 

 fiable. There are small blades that appear to be of artitioliie. One poor section. 



Age. Cabotian. N. H. w. 



No. 1208. GABBRO. 

 / 



About N. E. y sec. 23, Plummer (original Huronian area), Canada. 

 Ref. Annual Report, xvi, page 29. 



Meg. Similar to No. 1200. 



Mic. Less broken than No. 1200. There is in this rock much magnetite, some 

 sphene, hornblende, chlorite and spicular apatite. The green areas which seem to be 

 determined in outline by some earlier generation of a mineral now wholly changed, 

 show probably old olivines, and others augite. One (thick) section. 



Age. Cabotian. . N. H. w. 



No. 1264. BASALT. 



Rove Lake road, north of Grand Marais, near the south foot of Pine mountain. 

 Ref. Annual Report, xvi, pages 61, 119. 



Mic. The rock is porphyritic with fine feldspars that lie numerously in an 

 ophitic relation in the augite. There is developed considerable fine actinolite. The 

 augite is fresh, while the feldspars are much rotted. In crystallizing, the augites 

 apparently enclosed much of the magma glass in places, and this is now marked by 

 the sprinkling of magnetite grains in the augite crystals, but in other cases much of 

 the zirkelyte of the rock resulted simply from the condensation of the magma 

 without crystallization, and such portions are now thickly sprinkled with fine 

 magnetite particles. Between the nicols the rock in general is also much darkened, 

 apparently by the prevalence of fine zoisite particles with their high refractive index, 

 and by the generation of cltloritic substance. Two sections. 



Age. Keweenawan. 



Ili-mark. There seems to have been a long time (or a short period of great 

 activity) between the formation of the feldspars and the final consolidation of the 

 rock, the latter being but shortly after of contemporary with the formation of the 

 augites. During this intervening period not only were the olivines wholly lost by 

 resorption, but the feldspars were much affected by the introduction of many 

 microlitic impurities. N. H. w. 



