1882.] 
463 
[Merrill. 
products are common in pyroxenic or trachytic eruptive rocks. 
I certainly did not find quartz in “ comparatively large amounts.” 
A very interesting rock now claims our attention, Nos. 280 and 
281 among the dacites of the Report, concerning which Mr. 
Wadsworth enters into a detailed account, believing, as I under- 
stand him, 1 that the rock is not as a whole eruptive ; that the cement- 
ing material was “ water-deposited.” Zirkel has called the rock a 
“ dacite which envelopes so many strange fragments of another 
variety of dacite as to form a real breccia.” I think there can be 
no doubt that the rock is a somewhat brecciated eruptive rock, and 
that the cementing material is eruptive and not sedimentary. 
My reasons for that opinion are as follows : The quartzes of the 
main rock are numerous and generally contain glass inclusions. 
These glass inclusions are strikingly similar and prove, by 
their presence, that the quartzes are of eruptive origin, and 
indicate, by their similarity, that the quartzes had a common 
origin (i. e. belonged to the same rock and separated out under 
similar conditions). Now, these same quartzes of the main 
rock carry inclusions of a groundmass which appears to be iden- 
tical with that of the main rock, i. e., the cementing material. If 
only one or two of such inclusions occurred they might be only 
apparent inclusions, but really inlets and cavities of old quartzes 
belonging to some foreign, disintegrated rock, filled up with the 
new cementing material, but which had been so cut in preparing 
the section as to appear, in projection, as inclusions. But besides 
many evident examples of inlets of every form there were too 
many of such inclusions (sometimes a very small one well within 
the boundaries of a large quartz) to render such a construction 
plausible. Moreover, it would be remarkable if all of these 
cavities, penetrating far into the interior of a quartz, should have 
been washed out clean and refilled with the new cementing mate- 
rial. Therefore, I think that these quartzes are evidently original 
separations from the eruptive magma of this rock. The same thing 
is plainly visible in the quartzes of some of the enclosed fragments 
of another different eruptive rock. These quartzes also carry true 
glass inclusions and inclusions of the groundmass of that frag- 
ment very different from the groundmass of the main rock. I 
1 These Proceedings, Vol. xxi, p. 262. 
