38 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
very highly crystalline one, causing a rough-surfaced fracture. The sev- 
eral primary ingredients, except olivine, can nearly always be recognized 
with a lense, and in the coarser kinds with the naked eye. The specific 
gravity ranges from 2.8 to 3.1. 
The olivine, which is a common, but not constant, ingredient of these 
rocks, is, when present, always the oldest of the chief ingredients, as is 
plainly enough shown by its relation to the others in the thin section. 
Occasionally in the fresher rocks it may be detected with the lens in char- 
acteristic glassy, green grains, and at times is even of so large a size as to 
attract the unaided eye; as, for instance, in a resinous-hued, rather coarse, 
and a good deal weathered rock, which forms a low cliff on the north shore 
of Lake Superior (Sec. 34, T. 57, R. 3 E., Minnesota), a short distance east 
of the mouth of the Brulé River. In this rock the olivine occurs in abundant, 
black, glassy particles, from one-sixteenth to one-third inch in diameter, 
with a scaly structure from commencing decomposition, and is evidently, 
from its high iron content, close to the variety hyalosiderite. In specimens 
from the vicinity of Bladder Lake, in Ashland County, Wisconsin, a light- 
green, glassy olivine is very noticeable to the unaided eye. 
As seen under the microscope, the olivine occurs nearly always in 
irregularly-outlined, rounded particles, from a fraction of a millimeter to 
two or three millimeters in length. Only very rarely does it present crys- 
talline outlines. Commonly, it is largely fresh, presenting a grayish or 
nearly colorless section, with the characteristic rough surface. It is, how- 
ever, very rarely so fresh as to be without some traversing rifts, edged with 
a greenish-brown or brownish-yellow alteration-product. In less fresh 
kinds this brown alteration has affected the whole area, and in such cases 
the rock has macroscopically a pronounced resinous appearance. Less fre- 
quently, but still often, the iron oxide, instead of heing deposited about and 
within the olivine, has been leached out, and then the mineral is more or 
less completely represented by a greenish material, supposed to be serpen- 
tine. In a number of sections in which this greenish alteration was 
observed magnetite was noticed in small particles, associated with the 
green in such a way as to suggest that it also was an alteration-result from 
the olivine. In some of the very coarse-grained gabbros of Bad River, 
