12 RICHTHOFEN THE NATURAL SYSTEM 



Order Third : PropyWe. 



Family 1. Quartzose propylite. 

 " 2. HorribUndic propyltie. 

 " 3. Aug itic propylite. 



Order Fourth : Andesite. 



Family 1. Ilorrddendic andesite. 

 " 2. Augitic andesite. 



Order Fifth: Basalt. 



Family ] . Dalerite. 

 " 2. Basalt. 

 " 3. Leucitopliyre. 



Order First — Rhyolite. 



The name " rhyolite" was proposed, early in I860, 3 for certain rocks fre- 

 quently occurring on the southern slope of the Carpathians, and distinguished, in min- 

 eral character, from trachyte, which they otherwise resemble, by the presence of quartz 

 as an essential ingredient, and an almost infinite variety of texture. Beudant* had, 

 long before, described certain varieties of these rocks as porphyre trachytique, pumice- 

 stone, pearlite, etc. In 1861, the name " liparite" was proposed by J. Roth 5 for rocks 

 of similar nature occurring on the Lipari Islands. The term " rhyolite," however, 

 being of prior date, has since been almost generally adopted, among others by F. v- 

 Hochstetter, for rocks from New Zealand, by C. Peters, G. Stache, and others for those 

 of Hungary and Transylvania, by Ferd. Zirkel for those of Iceland, by B. v. Cotta as 

 a general term in his " Gesteinslehre." The word "rhyolite" is designed to express 

 one of the prominent features of these rocks. It is this : that their chief varieties 

 have the appearance, as it were, of natural glasses, and bear evidence, more than any 

 other rocks do, to the unpracticed eye, of having been flowing in a viscous state. 



Mode of Geological Occurrence. — Rhyolite has had its distinct epoch of eruption in 

 relation to other volcanic rocks. Wherever it occurs it may be easily proved to have 

 been of more recent origin than either propylite, andesite, or trachyte, but to have 

 preceded basalt in age. As to its geographical distribution, it is confined to the im- 

 mediate neighborhood of one or all of those antecedent rocks, and occurs ordinarily 

 within their veiy limits. Until lately, not much attention had been paid to it. But 

 since the establishment of the name, rhyolite has been found to be widely distributed, 

 though always occupying a subordinate position. In Hungary it usually skirts the 

 lower part of the flanks of andesitic ranges, forming hillocks and ridges of little 

 elevation, filling depressions, and issuing in currents from fractures, and, in general, 

 giving evidence of its entire dependency on the places of previous eruptions. 

 The greater portion of the rhyolite has, in that country, been evidently ejected 

 by volcanic activity. It appears that the same may be said in regard to the 



3 Loc. Cit. 4 Voyage en Hongvic. Paris, 1820. 5 J. Roth, die Gesleins-analysen. Berlin, 18G1. 



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