50 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
Tabulation of observations on coarse orthoclase-free gabbro and diabase—Continued. 
Angle between 
maximum ex- 
tinctions of 
! adjacent hemi- 
' tropic bands of 
5 : ; the plagioclase 
2 Pl | Macroscopic char-| Constituents as determined by in sections cut 
g BBD 2 acters. microscope, in order of age. at random in 
A 5 : the zone O: iit. 
q a et = 
o £ . = 
& |a|la : Angles on . 
4g = 2a aa ieee opposite og 
2 ep eel teal tel sides of| 4, 
a SClalal & cross-hair.| E © 
° ° ° 
1752.../Northwest shore |.....-. Sees ce ees tenes Medium-grained; | Olivine, very fresh, abundant, | 19 22 41 
Nipigon Bay, very light, gray, and large; apatite; labrador- | 16 17 33 
near mouth of mottled with ite; titaniferous magnetite; | 25 23 48 
Nipigon River, white; rough- augite, only slightly diallagic. 
Ontario, Canada. textured. Sp. A very fresh rock. 
gr., 3.10. 
Orthoclase-bearing gabbro—The rocks of this class are less common 
than those of the class last described, but are, nevertheless, often met with 
in all of the different districts crossed by the Keweenaw Series in its course 
around Lake Superior. They are distinguished from the ordinary gabbros 
and coarse-grained diabases by the presence of more or less orthoclase feld- 
spar, and of a plagioclase near oligoclase; by the.abundance of coarse- 
grained apatite, often large enough to be readily seen with a lens, or even 
with the naked eye; by the invariable absence of olivine; by the compar- 
atively greater tendency to decomposition of the feldspars, the results of 
which are a clouding and reddening of the constituents, and the introduc- 
tion of a greater or less amount of secondary quartz; by the greater rich- 
ness of the magnetite in titanic acid; and by the common presence of a 
uralitic alteration of the augitic ingredient. So constant is the association 
in the Keweenawan basic rocks of orthoclase, oligoclase, coarse apatite, 
secondary quartz and uralitic alteration of augite or diallage, that only in 
rare cases is one of these characters found without most of the others. 
In grain these rocks cover about the same range as those of the last 
class, running from a medium-grain to an exceedingly coarse one, when the 
feldspar crystals reach some inches in length. The specific gravity is 
lower—2.7 to 2.8—than with the more common orthoclase-free gabbros, 
Be 
