NOTES. 437 
breccias may, in part at least, have originated as volcanic ash, I imagine that it would 
be exceedingly difficult to prove such an origin for them. Constant avgularity of the 
particles might perhaps point that way, but a genuine vesicular character to each 
fragment would be about the only proof of such an origin, and in such ancient rocks, 
so profoundly altered as these must be, it would be extraordinary if angularity and 
vesicular character should be preserved. Dr. T. S. Hunt writes me that he observed 
nothing on Michipicoten Island that reminded him of the typical voleanic ash of such 
regions as those of Vesuvius and the Eifel, and suggests that these brecciated rocks may 
be due to the disintegrating effect of waters upon material extravasated beneath the 
sea. Such an origin is, of course, possible, and it may be true in a measure also of 
the common red sandstones of the Keweenaw Series, though these are very plainly in 
large measure composed of water-rolled and water-worn fragments. But, whatever 
the origin of the Michipicoten breccias may be, I have never met with anything just like 
them on either the north or south shores of Lake Superior, if Iunderstand Macfar- 
lane’s descriptions correctly. 
NOLE 
(Page 39.) 
PLAGIOCLASTIC INGREDIENTS OF THE KEWEENAWAN BASIC ERUPTIVES. 
Since this volume has been in type, a number of separations, by Thoulet’s specific 
gravity method, of the plagioclastic ingredients of the several kinds of Keweenawan 
basic rocks have been made by my assistant, Mr. C. R. Vanhise. These separations 
were undertaken with the view of determining whether only one plagioclase feldspar, 
as indicated by the optical method, or more than one, is concerned in the make-up of 
the Keweenawan basic eruptives. The investigation is not yet completed, but so far 
as it has gone it has tended to strengthen the conclusion already arrived at, namely, 
that only one plagioclase feldspar is ordinarily present, except in some of the por- 
phyritice kinds, in which the porphyritic plagioclases are different from the microliths 
of the groundmass. Especially in the case of the coarse olivine-gabbros and olivine- 
free gabbros did the experiment confirm the conclusions before arrived at, namely, that 
only one plagioclase is concerned, and that commonly lies, in its silica content, between 
anorthite and labradorite. The silica determinations made upon the separated plagi- 
oclases in no case showed less than 46 per cent. But if we accept, as it seems almost 
necessary that we should do, Tschermak’s theory of the nature of the intermediate 
plagioclase feldspars, this high silica content is easily explicable on the view that only 
one feldspar is concerned. Tschermak’s view is still further confirmed by the fact that, 
in every instance where the optical measurements stood near the border between 
anorthite and labradorite, the silica content also more nearly approached that of 
labradorite. 
See also Note 10 for the results of experiments made with the coarse anorthite- 
rock of the Minnesota coast. 
