20 RICHTHOFEN—-NATURAL SYSTEM 
Fam. lst. Sanidin-trachyte—The color of the paste varies, but it usually pre- 
sents light shades of gray, reddish, and reddish-brown ; its texture exhibits all the 
varieties mentioned. There are imbedded in it: crystals of sanidin, or of both sani- 
din and oligoclase, besides mica and hornblende ; the latter, however, is frequently 
wanting. To this family belong the rocks which compose the trachytic ranges of 
Washoe and Esmeralda, that of the Drachenfels, and many others. 
Fam. 2d. Oligoclase-trachyte.—The paste is of the same color as in the rocks of 
the first family, though darker shades prevail, and presents a similar variety of text- 
ure. Imbedded are, chiefly, crystals of oligoclase and hornblende, the former being 
frequently of the vitreous variety; the latter having usually the shape of broad 
needles, with a black color and bright cleavage-planes. Besides those minerals, 
black mica is of frequent occurrence. The rocks of this family are ordinarily associ- 
ated with those of the first subdivision, but in some localities are not accompanied by 
them. At Lassen’s Peak there is but one limited space where rocks of both families 
intermingle ; it is near the place where the lava has been ejected. Apart from it, the 
grand currents of lava, extending to from ten to twelve miles distance from the place 
of ejection, consist merely of oligoclase-trachyte. To this family belong those varie- 
ties of trachyte which were called ‘‘ domite” by L. v. Buch. 
The fact that the occurrence of the rocks of either one of these two families 
will frequently exclude those of the other, and that, even in those localities where 
they are associated together, they will occupy separate places in regard to geological 
superposition, appears to indicate that the distinction of these two subdivisions forms 
an approach to the requirements of the natural system. 
OrperR THIRD—PROPYLITE. 
The rocks of this order have hitherto occupied a very undecided position in 
the different classifications of rocks proposed, and just as various has been their nomen- 
clature when they had to be mentioned in geological descriptions. The fact that they 
bear close resemblance in mineral character to ancient diorite, while, geologically, they 
are intimately allied to voleanic rocks, has been the principal cause of this uncertainty 
of their position. In Hungary and Transylvania, they occur quite extensively, and, 
being of practical importance as the bearers of rich metallic veins, have had to be 
noticed frequently in treatises on the mines of those countries. Beudant applied for 
them the name ‘‘ porphyric greenstone,” and classified them, along with syenite, among 
fo) ’ ’ to) ? Oo 
they are both chiefly composed of oligoclase and hornblende, would render it indeed, practically, a very difficult and compli- 
cated task to compare the geological relations of different volcanic countries on the strength of written descriptions. If we 
now follow the classification of J. Roth, we find the application of the name trachyte limited to those voleanie rocks which 
contain sanidin, but are devoid of quartz. All those numerous varieties which do not contain sanidin, but are intimately 
allied to sanidin-trachyte, by their mode of geological occurence as well as by their physical characters, are excluded from 
the denomination, and, together with all volcanic rocks composed chiefly of hornblende and oligoclase, are united into one 
subdivision of andesite. The reasons which justify the separation of the compounds of oligoclase and hornblende into three 
different groups, (oligoclase-trachyte, hornblendie propylite, and hornblendic andesite) will be detailed in the following pages. 
It may then be understood why the adoption of the different systems of the current nomenclature would render the concise 
geological description of volcanic countries extremely difficult, and would conceal the harmony really existing in the rela- 
tions which they present in different countries. 
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