OF VOLCANIC ROCKS. 13 



rhyolitic rocks of New Zealand, St. Paul, and Iceland. But it has not been 

 so generally the case on the western coast of America. The volcano of Lassen's 

 Peak, and the environs of Mount Helena, in California, present grand instances 

 of a volcanic origin of rhyolite. But in the adjoining * State of Nevada it ap- 

 pears to have been extensively brought to the surface by massive eruptions. It is of 

 unusually frequent occurrence along the eastern slope of the Sierra Nevada, and far- 

 ther east in the Great Basin. Ranges of hills are there completely built up of rhyo- 

 litic rocks, not always in as close proximity to such as are antecedent to them in age, 

 as is the case in the Carpathians. 



It is one of the characteristic features of rhyolite, that it presents, more than 

 any other rock does, signs of having been in a state of what Daubree has called 

 "aqueous fusion," or the fusion of its mass by solution, under great pressure, in super- 

 heated water. Another peculiarity is the circumstance that the eruptions of rhyolite, 

 whether massive or volcanic, bear evidence of having been generally accompanied by 

 extremely violent solfataric action, which probably surpassed, on an average, that con- 

 nected with the ejection of other volcanic rocks. This action appears to have been 

 one of the chief agents in the formation of the rich silver-bearing veins of Hungary, 

 as well as of some in Mexico, and to have also been peculiarly characterized by the 

 occurrence of an unusually large amount of fluorine and chlorine among the escaping 

 gases. 



Mineral Composition. Rhyolite may be concisely defined as trachyte with an 

 addition of silica, not chemically combined, and which is either segregated into crystals 

 of quartz, or dissolved in the rock, and then no longer recognizable to the eye. It is, 

 owing to the high proportion of silica entering into its composition, the representative 

 of granite among volcanic rocks. Chemically, it is its complete counterpart, as far as 

 ascertained by analysis, and even in outward appearance certain varieties of rhyolite 

 offer at first sight a striking similarity with granite, though closer observation will at 

 once reveal well marked differences between both. Rhyolite outrivals any other rock 

 in respect to the truly astonishing number of its varieties, which are chiefly occasioned 

 by modifications of texture. It consists in general of a paste, with or without minerals 

 enclosed. 



The paste, chiefly, is liable to variation. Its colors are: white, gray, yellow, 

 green, red, brown, which occur in all manner of shades ; light ones prevail, while 

 perfect black has not been met with. The texture is as varied as the color. First 

 are to be noticed a number of hyaline varieties, which are represented by obsidian, 

 pumice-stone, and pearlite, and the frequent occurrence of which is a peculiar feature 

 of rhyolite. Though associated with volcanic rocks of every composition, these natu- 

 ral glasses, as they may be called, decrease in relative quantity and variety with the 

 decrease of silica. 6 Obsidian when having the composition of rhyolite, offers little 



6 There is no better example of tlie artificial principles on which the classification of rocks lias usually been 

 based than the fact, that accidental modifications of texture, which appear to result chiefly from the difference of the condi- 

 tions attending either the fusion or the cooling of the mass, have been considered as of equal value with other differences of 

 the greatest importance ; and pumice-stone, trachyte, basalt and pearlite have been considered as coordinate subdivisions, 

 even long after classification had been made dependent chiefly on mineralogical principles. 



a (51) 



