16 WIENS OWI INAUIL (HE GLE OLOGIG 
schistose. It is composed almost exclusively of colorless pyrox- 
éne and colored hornblende. Most of the latter mineral is of the 
character already so frequently described, but there is in addition 
a little bright green amphibole fringing areas of the brownish 
green variety. The fresh augite and the hornblende are both 
principally in small rounded grains closely packed together, pro- 
ducing a mosaic, polarizing brilliantly between crossed nicols. 
Locally in the section the augite granules unite to form a fairly 
compact plate of this substance. In one place a large nucleus 
of compact augite is surrounded by a broad irregular zone of 
small grains of exactly the same substance. The interior is 
crossed by many irregular lines that divide it into a large num-_ 
ber of small grains, but all of these are uniformly orientated, 
and between crossed nicols they polarize together. In the outer 
zone the individual grains are slightly removed from one another, 
and consequently each polarizes independently of all others. 
In a recent description’ of this rock it was intimated that the 
granulitic outer zone had been caused by the fracturing of the 
large augite grain and the movement of the broken portions from 
their original positions. A more careful inspection of the sec- 
tion teaches that this may not be the method of formation of 
the peripheral granulitic zone. Movement in the rock-mass 
during consolidation of the granules and posterior in point of 
time to the formation of the nucleus has certainly been instru- 
mental in causing the complex orientation of the granules in the 
outer zone, but that these latter represent fragments of an 
original large grain is not now believed. 
D. Conclusion. 
The above-described phases of the gabbro have been dis- 
cussed in more detail than their petrographical interest would 
seem to justify, mainly because their eruptive origin appears to 
have been called into question. 
The microscope has shown so conclusively that the rudely 
bedded basic and fine-grained dark rocks, interlaminated with 
™Geol. and Nat. Hist. Survey of Minn. 19th Ann. Report, p. 196. 
