DIFFERENTIATION OF KEWEENAW AN DIABASES 431 



which tend to become coarse grained and to separate into little 

 aggregations of feldspar and magnetite this texture is lost to a 

 large extent and the rock becomes more like a coarse gabbro. In 

 one place on the shore of Lake Nipigon some sand was collected 

 which showed poikilitic texture where feldspars were inclosed in 

 augite. 



In microscopical observations these rocks usually show labra- 

 dorite, augite, or diopside, and ilmenite or magnetite. Olivine is 

 widespread but is not always present and in specimens without 

 olivine quartz has been found, but it is lacking in many specimens. 

 Biotite appears in small quantities and titanite was found in one 

 section. Since the latter mineral occurs near a dike of acid rock 

 and is not commonly developed in diabases or gabbro, it is believed 

 to be due to the influence of this dike, as some of these acid dikes 

 carry titanite. 



Although these rocks are on the whole comparatively fresh, 

 certain alteration products occur. The olivine frequently shows 

 serpentine and iron oxide as alteration products, and the augite 

 and diopside, although usually quite fresh, often contain second- 

 ary amphiboles and actinolite. In a specimen from "Haystack 

 Mountain," north of Lake Nipigon, a crystal of magnetite occurs 

 partially surrounded by a mass of actinolite needles which, on 

 revolving the stage of the microscope, show 

 rotary extinction (Fig. 1). These needles 

 seem to be the product of alteration of an 

 augite crystal whose growth began around 

 the magnetite and they resemble similar 

 fibrous growths which W. S. Bayley de- 



., . , Fig. 1. — Magnetite par- 



scribes as occurring around magnetite in tiaUy surrounded by augite 

 the basic rocks of the Lake Superior region, which has altered to actin- 

 although he does not ascribe a secondary olite needles (greatly en- 

 origin to them. 1 In a. specimen from the arge 

 shore of Lake Nipigon, opposite "Two Mountain" Island, the 

 diopside and magnetite are intergrown to some extent and the 

 latter sometimes occurs as a fringe along the border of crystals of 

 the former. Although much of the magnetite associated with the 



1 Journal of Geology, I, 702-10. 



