OF VOLCANIC ROCKS. 23 



grained propylitic 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, microcrystalliue 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, Rocks 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 



(61) 



