1881 .] 
241 
[Merrill. 
solely upon the general habitus of the specimens entirely with- 
out regard to their geological relations ; and Professor Zirkel 
himself expressly states that he entertains similar views on page 
101 of his report, concerning specimen No. 200. But one would 
scarcely be able to infer from Mr. Wadsworth’s laconic attacks 
that any such qualifying concession whatever had been made. 
The writer proceeds : “ The iiropylites [i. e., of Zirkel’ s report] 
are all altered andesites, with which species their chemical com- 
position agrees ; and the diagnostic distinctions that Professor 
Zirkel has jilaced between the andesites and propylites do not 
hold good even in the specimens that he described, as would 
have been readily seen had he given complete descriptions instead 
of the very imperfect and often inaccurate ones that have been 
published.” It seemed to me that the chemical compositions of 
the andesites and propylites, as given in Mr. King’s tables of 
analyses, correspond significantly in their slight differences to just 
the diagnostic distinctions drawn by Professor Zirkel, some or all 
of which were generally visible to me under the microscope. 1 
The writer progresses (p. 285) : “ All of the dacites, [i. e. of 
Zirkel’s report] with one exception, are rhyolites, felsites, and 
fragmental rocks not beh aging to andesites. The exception is 
an altered andesite, in which the quartz is an alteration product.” 
Here, as in all of the tlier cases, we are not told which ones 
are the rhyolites, which the fe 1 sites, or which the one with the 
alteration product, quartz. This, doubtless, will all be done in 
1 Although. Mr. Wadsworth refers to no page, yet I take the diagnostic distinctions 
between the propylites and andesites, to which he refers, to be those cited on pages 
132 and 133 of the printed report of Professor Zirkel. As bearing upon the question 
of chemical composition, the most important diagnostic distinction between the pro- 
pylites and the hornblende-andesites is probably that of the dissemination of the horn- 
blendic (and epidotic) dust in the groundmass and feldspar of the propylites, amount- 
ing in some cases to a real saturation of the feldspars with this material, — and the 
usual lack of this in the hornblende-andesites. It seemed to me that slides correspond- 
ing to the numbers 212,213, 218, 219 of the printed report (108, 110,156, 157 ‘‘coll, 
numbers ”) showed this distribution of hornblendic matter very well. This sprink- 
ling of hornblendic dust in the feldspars in addition to the hornblende separations 
proper to the rock would have a slight tendency, naturally, to lower the percentage of 
silica and to raise that of magnesia. This is especially true concerning the silica, 
6ince orthoclase separations thus laden with hornblendic material are not very infre- 
quent. In the tables of analyses published by Mr. King, the average per cent of silica 
is some 1.47 lower in the propylites than in the andesites, making no correction for 
PROCEEDINGS B. S. N. H. VOL. XXI. 16 FEBRUARY, 1382. 
