W) CESIIPSLY RG TES. _—_ aeet E e E een  T 
é 
4 
ORTHOCLASE-FREE GABBRO, 41 
areas which contain these needles, and the irregularity and frequent great 
breadth of the lineations, as a result of which crystals are found at times 
giving only two bands, have led to the idea that such rocks from other re- 
gions contain orthoclase,’ and the same conclusion has been announced for 
the Bad River gabbro by Julien,” who also supports his decision by 
an appeal to the cross-barred twinning of the grains as seen in the polarized 
light. This appearance was for a long time supposed to be characteristic of 
orthoclase, but the supposed orthoclase has since been shown to be micro- 
cline, and a cross-barred twinning is now known to be common in labradorite 
as well. Moreover, a careful examination of the cleavage directions, so often 
emphasized by inclusions, makes it certain that we have to do with a tri- 
clinic feldspar. The plagioclase of these rocks is often the most plentiful 
ingredient, but in the darker colored varieties is dominated by the augite, 
which in some of the very black kinds constitutes nearly the whole section. 
The iron-oxide constituent of these rocks appears commonly to stand 
between the plagioclase and the augitic constituent in point of time of erys- 
tallization. This is not always evident, even for the magnetite that is a pri- 
mary constituent, while there is undoubtedly at times a magnetite resulting 
from the alteration of the augite. The distinction between magnetite and 
titanic iron, still more between magnetite and titaniferous magnetite, in rock 
sections, is a difficult one always, and often is well-nigh impossible without 
a quantitative analysis, when the characteristic crystalline outlines of mag- 
netite and the equally characteristic white alteration-product of titanic iron 
and of highly titaniferous magnetite are both lacking. The powder of the 
finer grained of these rocks generally yields a considerable beard to the 
magnet, and in the coarser kinds the plainly visible metallic-lustered part- 
icles are always strongly magnetic. A number of qualitative tests made 
yielded, about half and half, negative results and feeble reactions for titan- 
ium. On the whole, I am inclined to consider that in these rocks the iron- 
oxide ingredient is always a titaniferous magnetite, although the titanium is 
at times in very minute quantity. In size this ingredient runs from mere 
dust to particles a quarter of an inch across, and usually is. in as large 
1H. Rosenbusch, op. cit., p. 460. 
2Geology of Wisconsin, Vol. III, p. 234. 
