DIABASE-PORPHYRITE. ii 
Tabulation of the results of a microscopic study of fine-grained olivinitic diabases (includ- 
ing melaphyrs)—Continued. 
| | Angle between 
| maximum ex- | 
tinctions of 
adjacent hemi- 
| tropic bandsof | 
8 | the plagioclase | 
2 ives a Macroscopic char- Constitutents as determined by in sections cut 
g : a acters. microscope, in order of age. at random in 
5 S “| | the zone O: 77. 
A a A= | 
® eB :\a f | Angles on| © . 
=| & eat 
E 2 i/sia|] gs opposite | os 
= € 3S E g aden of| Ha 
E ejin'ial & cross-hair.| i 
| } } | | °o ° °o 
958....| North shore Lake | SW. | 11 | 60 | 2W. Fine-grained; | Olivine, abundant, in small | 34 | 32 66 
Superior, Cari- | dark reddish- grains, wholly altered to a red 32 35 67 «| 
bou or Black brown. substance and crowded into 25 | 30 55 
Point, Minne- | theaugiteinterspaces; anorth- | 35 | 34 69 
sota, from a) | ite; magnetite; augite, in the | 
layer overlying | | usual fresh areas; an exces- 
red sandy shale. | sively fine-grained rock. | 
| 
Diabase-porphyrite and ashbed-diabase-—The olivine-free, fine-grained 
diabases of the ordinary type pass into still finer-grained kinds, in which 
there is a black or brown color and a more or less perfectly developed con- 
choidal fracture. The finest of these rocks are completely aphanitic, and 
all kinds tend to a porphyritic development, carrying, as porphyritic ingre- 
dients, oligoclase and orthoclase, and, more rarely, labradorite and augite. 
The augite is always a subordinate ingredient, and appears at times to be 
almost wholly absent, at least as an individualized substance. These rocks 
play a very prominent réle in the Keweenaw Series, and are always very 
strongly characterized in the field. 
Some of the more distinctly crystalline of these rocks, as already stated, 
Pumpelly has described under the name of ashbed-diabase, the name being 
given from the fact that such a diabase forms the base of a flow, whose upper 
vesicular portion is the well-known and so-called ashbed of Keweenaw 
Point. As the external characteristics of this class of rocks, Pumpelly gives 
a light or dark gray or black color, a very compact texture, and a conchoidal 
fracture, and, as the accompanying microscopic characters, the subordinate 
position of the augite, and more especially its occurrence in rounded grains, 
whose contours are not determined by the feldspars.’ 
1Geology of Wisconsin, Vol. III, p. 32. 
