Merrill.] 
236 
[October 19, 
upon the microscopic petrography of the Fortieth Parallel collec- 
tion, Professor Zirkel, in describing and illustrating his use of the 
word microfelsitic , writes almost the identical words quoted 
above, remarking expressly that the microfelsitic base is of rare 
occurrence among the basic rocks. What, then, are we to under- 
stand by Mr. Wadsworth’s “ original microfelsitic base of the 
unaltered andesites ” ? 
Since the introduction of the term, microfelsitic into petro- 
graphic literature, writers have certainly used the term somewhat 
confusedly, but this is irrelevant to the point at issue. The un- 
welcome thought intrudes itself upon the mind of the reader of 
Mr. Wadsworth’s paper, that that gentleman cannot be familiar 
with the microfelsitic base, in the Zirkel sense. This is suggested 
here, there and everywhere ; for example, again, on page 281 he 
alludes to the “ clear glassy base ” of rhyolites and speaks of a 
devitrification of this glassy base in the older and more altered 
rhyolites as giving rise to the “ microfelsitic (so-called) base, that 
has so puzzled lithologists in the study of the felsites. ” Now, 
besides the purely glassy varieties of rhyolite, like obsidian, pear- 
lite, etc., rhyolites certainly occur with a glassy base ; but the 
writer had no need to go back to the older felsites to arrive at a 
microfelsitic base, since it is precisely in the late rhyolites them- 
selves where it is typically developed. I have dwelt somewhat at 
length upon these considerations since their connection with the 
rest of this paper is obviously intimate and of some significance. 
On page 279 the author continues: “The fine-grained, compact 
basalts through their alteration give rise to the variety known as 
melaphyr, while the coarser-grained ones, like the dolerites, in like 
manner form the diabases and the great majority of the basic 
rocks known as diorite, — a name that is at present the most 
abused one that occurs in lithology, as it is made to cover, not 
only old basalts, andesites, etc., but also the granites of the Sierra 
Nevada. May it soon be dropped!” There is to be noticed a 
slight inaccuracy, or rather incompleteness in the statement that 
the coarser-grained basalts, like the dolorites, give rise to the dia- 
bases by alteration. The older equivalent of dolerite is hardly 
diabase proprement dit , but rather olivin-diabase ; while diabase, 
pure and simple, is the older equivalent of augite-andesite ; al- 
