ROCKS OF NIPIGON AND BLACK BAYS. 337 
dip very plainly brought out in a long front slope and precipitous back 
slope, and at the same time a most beautifully developed columnar structure. 
Some of these islands must be remnants of very heavy layers, the columnar 
back cliffs sometimes considerably exceeding a hundred feet in height. 
Imay add the results of a microscopic study of a few of the specimens 
collected by me from this region. 
A rock from the cliff on the southeast shore of Black Bay is very fine- 
grained and black, with sparsely scattered and minute true vesicle-fillings 
of calcite, quartz and chlorite. The thin section shows a groundmass con- 
sisting of a dirty brownish-white, impellucid isotrope material (altered glass), 
with tabular feldspar microliths. In this are included as porphyritic ingre- 
dients abundant areas of augite, each made up of a number of detached 
grains. Small plagioclases (oligoclase) also oceur porphyritically. The 
vesicles filled with the minerals named above are seen to have sharply 
defined outlines, and to have the material immediately about them more 
dense than the rest of the rock. Chlorite also oceurs in pseud-amygdaloidal 
areas, when it is an alteration-product of the augite. The rock is somewhat 
peculiar from its great abundance of augite. 
Another rock from the same cliff farther to the southwest is finely 
crystalline, and black, with a very rough, lumpy fracture. Under the 
microscope it proves to be a very highly augitic diabase of the ashbed type, 
and is plainly a non-amygdaloidal phase of the last described rock. 
About a mile still farther southwest the east shore of Black Bay shows 
fifteen feet of red- and white-mottled and striped, cross-laminated sandstone, 
underlying black diabase The thin section of this sandstone shows that 
itis chiefly made up of angular and sub-angular quartz grains, which oceur 
of two sizes, the prevailing small ones constituting a sort of matrix in which 
the large ones float. Large-sized particles of orthoclase, microcline and 
oligoclase are also noticeable in a tolerably fresh condition; and a fine 
cloudy material in the base is probably comminuted and decomposed feldspar. 
The red blotching is due to the iron staining. The sandstone is plainly 
made up of granitic débris and not of the usual porphyritie detritus, a fact 
which is easily explained by the proximity of this sandstone to the granites 
of the west and northwest sides of Black Bay. 
22L Ss 
