OLIVINE-FREE DIABASE. 63 
medium fine-grained to erypto-crystalline, and the fracture from uneven 
and hackly to conchoidal.”* To this I have only to add that a purplish 
shade, varying from a brownish-purple to a bright reddish-purple, is a very 
common one, and that a true conchoidal fracture is characteristic only of 
the kinds here set apart as a separate group under the name of ashbed- 
diabase. 
The pseud-amygdaloidal alteration of these rocks, while it has taken 
place largely in the true amygdaloid, and has commonly affected the entire 
thickness of a bed in some degree, is especially characteristic of the middle 
portions of the bed. The common pseud-amygdules are chlorite, quartz, 
prehnite and calcite. They vary greatly in size, running from a quarter 
of an inch or more—rarely several inches—in diameter, down to minute 
particles. In the former case the rock is coarsely blotched with the colors 
of the pseud-amygdules, looking, as said, like a true amygdaloid, while in 
the latter case the only effect on the external appearance is a variation of 
the general shade. 
Under the microscope, when not too profoundly altered, this diabase 
‘is seen to have for primary constituents plagioclase, augite and an opaque 
black mineral, which may be either magnetite or titaniferous iron ore.” 
The last-named ingredient never shows any distinct crystalline outlines. 
It appears generally to be strongly attracted by the magnet, while, judging 
from the results of the analyses cited by Pumpelly® of the rocks of several 
beds of the Eagle River section of Keweenaw Point, titanic acid is often 
present. In these rocks, then, as in the coarser kinds previously described, 
the iron-oxide constituent appears to be a titaniferous magnetite. 
The plagioclase appears, from optical measurements, to belong near oligoclase in 
the feldspar series. It appears in tabular polysynthetic crystals, whose long, narrow 
sections are scattered confusedly through the section, while the spaces between the 
crystals are occupied by augite, the augite in each space generally giving the integral 
polarization which indicates a single individual. In ordinary light the augite is dis- 
tinguishable from the plagioclase by its very faint, delicate, violet-gray color, and by 
its anastomosing cracks. The sharpness with which it fills the interstices between the 
feldspar crystals shows that it crystallized after them. The magnetite occurs in small 
grains, rarely with an appearance of crystal outlines.‘ 
'R. Pumpelly, Geology of Wisconsin, Vol. III, p. 31. 
2R. Pumpelly, Geology of Wisconsin, Vol. III, p. 32. 
3R. Pumpelly, Metasomatic Development, pp. 285-293, etc. 
4R. Pumpelly, Metasomatic Development, p. 270. 
