20 RICHTHOFEN — NATURAL SYSTEM 



Fam. 1st. 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. 



Order 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 volcanic 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 



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 volcanic 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, arc 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, hornblendic 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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