It has seemed to me that it would be well in connection with the following paper, which should be read first, on porphyritic appearance to show how the principles and results there stated are used in particular cases. I propose, therefore, in this paper, in the first place, to study the grain of the augite in a group of chemically similar diabases. This is the mineral which seems to lend itself best to mathematical treatment. The comparative results are interesting and suggestive. I will, then, take one of these rocks in Marquette and study the grain of all its constituents to show how we must use our formulæ, with due regard to petrographic observations and scientific common sense.
But first will be illustrated the methods of computation in the case of one rock, which I take to be effusive, the (greenstone) ophite of the Isle Royal report,* which is . . .