PEEVIOUS mVESTIGATIONS. 27 



site, rhyolite, and a strange variety of basalt to the list of rocks previously 

 recognized. Professor Zirkel formulates the diagnostic differences between 

 propylite and andesite as follows:^ 



propyiite. — "« The general color of the propylitic groundmass has more 

 of a greenish-gray, while the andesitic groundmass has more of a pure gray 

 or brown tinge. 



"&. In structure and in the behavior of its constituents, the propylite 

 still resembles the older ante-Tertiary diorite-porphyries. 



"c. The groundmass of the propylites is rich in minute particles of 

 hornblende, while in that of the andesites this mineral appears only in the 

 larger individuals, fine hornblende dust being wanting. 



"rf. The propylitic feldspars are usually filled with a considerable 

 quantity of hornblende dust, while the andesitic feldspars are entirely with- 

 out it: the latter not infrequently containing glass-inclusions, which do not 

 seem to occur in the propylitic plagioclases. 



"e. The color of the proper hornblende sections in propylite is always 

 green (never brown), while the color of those in andesites is almost with- 

 out exception brown ; and the propylitic hornblende never shows the curious 

 black border which is so common to that of andesites; and again, propylite 

 in some cases contains, besides the largely predominating green hornblende, 

 a few sections of the brown mineral, presenting, in many points, a strikingly 

 peculiar aspect, while in andesites two kinds of hornblende never occur 

 together. 



"/ The propylitic hornblende is often distinctly built up of thin 

 needles or staiF-like microlites, and therefore is not regularly cleavable ; 

 which has never been found to be the case in andesites. 



"</. The production of microscopical epidote (mainly by the alteration 

 of hornblende), so very common in propylites, has, with one exception, 

 never been observed in these andesites, and it is also unknown in the Eiu-o- 

 pean occurrences. 



"/i. Augite often occurs as an accessory constituent in andesites, but 

 it is comparatively veiy rare in propylites. 



"^. The andesitic groundmass here and there seems to possess a half- 

 glassy development : a glass-bearing propylitic groundmass has never been 



1 Exploration of the Fortieth Parallel, Vol. VI., p. 132. 



