THE 
GEOLOGICAL MAGAZINE. 
NEW SERIES. “DECADE Vi VOER Xx: 
No. IV.—APRIL, 1883. 
OmLiG- ti AT AR EeleCLhasS-: 
I.—Notes on THe Cunviot ANDESITES AND PORPHYRITES. 
By J. J. Harris Tratz, M.A., F.G.S. 
[PLATE IV.] 
(Continued from p. 108.) 
he proceeding to describe the more common porphyrites, I 
wish to return to the subject of the true andesites, referred 
to in my last paper, and their relation to Tertiary and recent rocks 
of a similar chemical and mineralogical composition. On referring 
to the note appended to that paper, it will be seen that my 
description of the pyroxenic constituents of these rocks was very 
imperfect, not to say inaccurate. The number of elongated sections 
which extinguished parallel with the vibration-planes of the crossed 
Nicols attracted my attention when first I examined the rock, but 
seeing that the same feature occurred in the so-called augite-andesites 
of Hungary and Santorin, and seeing further that as regards cleav- 
age and crystalline form the pyroxenic constituents of the Cheviot 
rock resembled those of the augite-andesites above mentioned, I 
came to the conclusion that the prevailing pyroxene in both cases 
was the same, and also that it was augite. In the former conclusion I 
still think I was right, but in the latter I was unquestionably wrong. 
A flood of light, as far as I am concerned, has been thrown on 
this question by an article in the February number of the American 
Journal of Science, by Mr. Whitman Cross, to which my attention 
was called by Prof. Judd. The article is an abstract of a paper to 
be published as a Bulletin by the U. 8. Geological Survey, and is 
entitled “On Hypersthene-Andesite.” It commences as follows: “ In 
the course of the investigation of some apparently normal augite-ande- 
sites of the most typical variety, occurring at the Buffalo Peaks in South 
Park, Colorado, the writer found that a large part of the pyroxenic 
constituent possessed the crystalline form and chemical constitution of 
hypersthene rather than of augite. The comparative study of similar 
andesites from this country and from well-known European localities 
has forced him to the conclusion that in very many, if not in all of 
them, augite is decidedly subordinate to a rhombic pyroxene, which 
is presumably hypersthene.” After giving a description of the 
general macroscopic and microscopic characters of the Buffalo Peaks 
rock which would answer very well for the Cheviot andesite, he 
states: “When the pyroxene crystals and grains . . . are examined 
in polarized light, it is clear that a large portion of them do not 
belong to the monoclinic augite. If, in the first place, all those 
individuals of which the vertical axis seems to lie in, or nearly in, 
DECADE II.—VOL. X.—NO. IV. 10 
