PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 375 



Diabase.] 



Olii'ine is not abundant, but preceded the feldspar. It is much darkened by 

 magnetite, and somewhat by chlorite and also by bowlingite. 



Augite preceded the feldspars in large part indeed, but little or no augite can 

 be seen that followed the feldspars but it is much altered, some of it having given 

 place to a cloudy, brown and confused mass which cannot be differentiated, but 

 apparently consisting of chlorite and hematite principally. This alteration of the 

 old augite seems to have been cotemporary with the formation of several other 

 minerals, viz.: 



Quartz, hornblende and biotite, which appear as secondary minerals, but not in 

 sufficient amount to seriously interfere with the classification of the rock. 



Apatite is abundant, cutting the feldspar and the quartz. 



There is a small amount also of what may be considered a devitrified glassy 

 residuum from the magma, although these areas are so similar to the decomposition 

 products from the augite that they can hardly be distinguished. One section. 



Age. (Cabotian?) dike in the Animikie(?) 



Remark. It is the most reasonable to refer this rock to the age of the great 

 eruptive which forms the hills adjacent, which has been called a part of the gabbro. 

 It has also certain petrographic alliances with the great dikes of the Grand Portage 

 region, especially in the fact that the augite preceded the feldspars, a character 

 which has not, as yet, been known to be prominent except in the Grand Portage region. 



As to the date of the alteration of the augite, and its cause, see special discussion 

 of petrographic peculiarities in Part III. N. H. w. 



No. 460. DIABASE (with olivine). 



Diabase from dike above the lower falls of the St. Louis river, nearly opposite Island No. 6; N. E. J^ S. 

 W. 34 sec. 10, T. 48-16 W. From a smaller dike -probably a dike ten feet in width. 



Kef. Annual Report, x, page 13; Final Report, vol. iv, plate A, and pages 17, 571. 



Meg. A rather fine-grained, heavy, black diabase with a resinous luster. 



Mic. The section shows a diabase composed essentially of lath-shaped plagio- 

 clases, augite and magnetite. The plagioclase shows equal extinction angles on either 

 side of the twinning plane running up as high as 30; a grain which gave a positive 

 bisectrix had an extinction angle of 23; both of these determinations indicate 

 labradorite. The augite is mostly later than the labradorite, though some grains 

 seem to be as early or earlier than some of the labradorite. The rock, especially the 

 augite, has been altered considerably in places, and as secondary minerals there are 

 hornblende, chlorite, biotite, magnetite and quartz. There are also some yellow areas 

 (bowlingite ?) which perhaps represent original olivine, and a few grains of olicine 

 altering to a green product are present. One section. 



Age. (Cabotian ?) dike in Animikie(?) rocks. u. s. G. 



