1882 .] 
469 
[Merrill. 
structure, which is also not uncommon in rhyolites. The thin 
sections showed no olivine or augite (unless some of the microlites 
are augitic) ; they carry hornblende besides the mica, and the 
sanidins with fine zonal structure carry glass inclusions. The 
rock microscopically and macroscopically has not the faintest 
shadow of a resemblance to a basalt. Yet Mr. Wadsworth finds 
that the handpiece is “evidently a basalt.” 
Mr. Wadsworth thinks 1 that No. 483 is evidently an andesite, 
as Emmons suggests. I did not find quartz in the rock, and as 
there is much plagioclase besides the sanidin, and since the 
groundmass is, as Zirkel noticed, like the characteristic groundmass 
of augite-andesites, I should think that this might be called not 
improperly an augite-an desite. Zirkel remarked that the rock 
was of a quite different type from rhyolites just previously de- 
scribed. 2 
One could wish, in passing, that a further elucidation had been 
given of Mr. Wadsworth’s remark : “ The quartz found in the 
rhyolitic groundmass is regarded by me as a devitrification pro- 
duct.” 3 
Under basalts I agree with Mr. Wadsworth in considering 
(on purely lithological evidence) No. 529 as a diabase. 4 It seems 
to me to be thoroughly crystalline ; the feldspars are in pretty 
large ledges and there is much green (augitic ) alteration pro- 
duct. But Professor Zirkel has remarked 5 that the basalts from 
this region (American Flat Canon, Washoe) are not characteristic 
tyjoes and present exceptions to the general behavior. Mr. Wads- 
worth does not make mention of this remark. 
No. 531 is considered by Mr. Wadsworth to be “ an old, altered 
and much decomposed andesite (propylite),” and he adds : “ This, 
both macroscopically and microscopically, is identical with some 
of Zirkel’s propylites, and contains some quartz. Zirkel mistook 
for bnacroscopical olivines’ some greenish (viriditic) spots arising 
from the alternation of the groundmass in the rock! Its chemical 
analysis also indicates that it belongs to the andesites.” 6 Pro- 
1 These Proceedings, Vol. xxi, p. 269. 
‘ 2 Report, p. 188. The number is given incorrectly on p 188 as 423. It should be 433. 
3 These Proceedings, Vol. xxi, p. 269. 
4 Ibid., p. 270. 
5 Report, p. 230. 
6 These Proceedings, Vol. xxi, p. 270. 
