Wadsworth.] 
262 
[October 19, 
a real breccia” (VI, pp. 140, 141 ; Nos. 280 [1866], 281 [1884]). 
This dacite the writer regards as identical with specimens 
obtained by Mr. W. A. Goodyear from a quarry near the Manhat- 
tan Mercury Mine, Johntown, Yolo Co., California, now in the 
Lithological Collection of the Museum of Comparative Zoology. 
These specimens are composed of fragments of pumice (the 
•‘strange angular particles of a dull, milky looking substance” 
of Zirkel), andesite, quartz, feldspar, etc., cemented by a fine mud 
and water-deposited silica. The rock is composed simply of de- 
tritus that has been somewhat acted upon by water, etc., and 
belongs with the clastic rocks. 
It may be remarked that these rocks are about as unlike the pro- 
pylite of Richthofen as any rock could well be. It is perhaps 
permitted to me to speak somewhat authoritatively upon this point 
since there is under my charge a collection of propylites, deter- 
mined as such by Richthofen, from the typical localities of Silver 
Mountain, Washoe, etc. 
But to continue, King states of a rock from another locality 
(I, pp. 569, 570) : 
“ Of all the dacites, in external habitus this most closely resem- 
bles the propylite type , and it is by mistake colored upon our geo- 
logical map as quartz-propylite , close examination having been 
made too late for a change.” The analysis, too, is given in the 
table of analyses No. VIII (analysis No. 132) amongst the 
quartz-propylites, and not in Table IX where Mr. King states it 
is to be found. 
Speaking of this dacite of Professor Zirkel, Mr. Hague remarks 
(II, pp. 844, 845) : “ It is regarded as belonging to the quartz-propy- 
lites , and has been represented as such on the geological maps, al- 
though microscopical analysis indicates that it is more closely allied 
to dacites, the quartz bearing variety of the andesites. The rock 
has the characteristic greenish-gray groundmass of the typical 
Washoe propylite , with the same arrangement and structure of 
the plagioclase and hornblende and the same general field aspect .” 
Yet this is a rock that Zirkel describes as “ one of the most typical 
dacites ” (VI, p. 139, No. 276, Col. No. 814). It is regarded by me 
as an old rock probably long antedating the Tertiary, and would 
by most lithologists be called a quartz or a granite porphyry. 
