798 THE GEOLOGY OF MINNESOTA. 



[Gabbro. 



the augite itself, and that instead of being later formed, as presumed by either of 

 the above hypotheses, it is really first formed. At the same time the augites in this 

 rock are occasionally somewhat zoned. One section. 



Age. Cabotian(?) N. H. w. 



No. 1848. GABBRO. 



Same place as the last. 



Ref. Annual Report, xxii, page 9. 



Meg. Massive, pyroxenic rock, apparently in form of a dike cutting No. 1847. 



Mic. This rock is quite like the last, which indicates it was cotemporary with 

 it. The augite is partly altered to hornblende, showing narrow fringes of the latter 

 extending beyond the augite boundaries. There are also small areas of hornblende 

 isolated wholly from all augite. Such are frequently ensconced in a patch of green 

 alteration products which may be delessite, and narrow needles are scattered in the 

 same material, accompanied by epidote. The augite is frequently twinned on 100. 

 Olivine, which is wholly changed, remains only in the shape of sub-rounded masses 

 of green (antigorite?), which are often wholly surrounded by a single augite crystal. 

 The feldspars are likewise so altered that they can hardly be discerned. It is 

 apparent that there were no feldspars of the first consolidation, the only feldspathic 

 forms visible being quite small, and mingled with the alteration products. As the 

 augites are substantially idiomorphic, embracing only the older olivines, and imping- 

 ing only on each other, they seem to have been earlier to form than any feldspars, 

 which gives the rock one of the characters which, by some, has been considered 

 essential to the rock gabbro. It certainly was not ophitic. One section. 



Age. Cabotian(?) N. H. w. 



No. 1849. GABBRO. 



Dike cutting the same island diagonally. 

 Ref. Annual Report, xxii, page 10. 



Meg. Porphyritic with feldspar. 



Mic. The older feldspars are not abundant, and the small augites were about 

 cotemporary, or slightly earlier than, the second generation of feldspar. In the 

 older crystals, sections perpendicular to n s have an angle of extinction on the basal 

 cleavage of 9 to 12, * being the acute bisectrix, indicating an andesine, with a 

 tendency toward labradorite. Such crystals are in general clouded by alteration, 

 and occasionally indistinctly zoned. The small feldspars, however, are almost 

 without exception zoned, the central parts being charged with sericite. The augite 

 is very largely uralitized and the olivine serpentinized, and magnetite accompanies 

 these alteration products. This rock was in part ophitic, as now revealed by the 

 distribution of the alteration products of the augite, showing also two periods of 

 formation of augite. One section. 



Age. Manitou(?) N H w 



