OF VOLCANIC ROCKS. 93 
grained propylitie detritus and massive propylite, and the entire system is intersected 
by dykes of the latter. 
A particular interest, which has a practical bearing of some importance, 
attaches to propylite, inasmuch as, notwithstanding its limited occurrence, it yields 
probably a larger amount of silver than any other rock. Several of the principal 
silver-bearing veins, as the Comstock vein in Washoe, the celebrated veins in Hungary 
and Transylvania, as well as some of those in Mexico, and probably too in Bolivia, are 
enclosed in propylite. 
Mineral Composition —In propylite are united the petrographical properties of 
ancient dioritic rocks with those of andesite and oligoclase-trachyte. Varieties are 
numerous, but produced more frequently by the difference of the component minerals in 
size and relative proportion than by the texture, which remains porphyritic in every in- 
stance. The most common varieties consist of a fine-grained, microcrystalline paste of 
dark green or greenish brown, more rarely of reddish and dark gray colors, in which 
are imbedded crystals of oligoclase and hornblende ; the former is of whitish or light 
green color, the latter ordinarily dark green and fibrous, seldom black with bright 
cleavage-planes (as is the case with gamsigradite). The paste appears to be a fine- 
grained aggregation of the same two minerals (the feldspathic ingredient prevailing), 
with the admixture of titanic iron, and to owe its ordinarily green color to the profuse 
dissemination of small particles of green, fibrous hornblende. Sporadic crystals of 
augite are occasionally met with in some of the common varieties, while others contain 
rounded grains of quartz sparsely enclosed. Recent observations have led to the dis- 
covery of two series of varieties deviating from the usual composition as explained. 
One of them is produced by the increasing proportion of the grains of quartz; the 
other by the more profuse occurrence of augite. The former was found by Stache, in 
Western Transylvania ; the latter observed by me at Silver Mountain. Both appear 
to be of a limited geographical distribution, but are of great interest, as they extend 
the limits of the order of propylitic rocks, without rendering either their petro- 
graphical character, or their geological relations less distinct. In regard to the latter, 
it may be stated that the entire range of the quartzose to augitic varieties was anterior 
in age to andesite. 
Difference of Propylite from other Volcanic Rochs nearly related to it—The rocks of 
two other orders have an affinity to propylite, namely oligoclase-trachyte and ande- 
site. The principal ingredients of either are hornblende and oligoclase, and either of 
them contains crystals of both these minerals imbedded in a paste. Yet there is a 
notable difference in character between these two, as well as between either of them 
and propylite. It is so conspicuous to the eye that the respective rocks have ordi- 
narily been distinguished in geological descriptions, even in those of older time ; and 
even the unprofessional eye would be able to distinguish the three groups in a collec- 
tion of specimens belonging to them. Yet, it escapes description. It may, at this 
present time, safely be founded on what the botanist would call ‘ habitus,” a certain 
general character which it is as easy to recognize by the eye as it is difficult to describe 
it in words. and impossible to define its causes. It is probable that observations, such 
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