14 RICIITHOFEN NATURAL SYSTEM 



difference in character from that which is a modification of other volcanic rocks. In 

 regard to pumice-stone, however, Abich has proved that when formed of the material 

 of trachyte or andesite it has rounded pores, and ordinarily a green tint ; while those 

 varieties which have the composition of rhyolite, excel by the elongated and irregular 

 shape of their cavities, which are enclosed in a fine tissue of fibres of silken appearance 

 and white color. Between both kinds of pumice-stone there are gradations apparently 

 dependent as to their character on the amount of silica entering into their composition. 

 Pearlitic texture is peculiar to rhyolitic rocks. From these more or less perfectly glassy 

 varieties there are gradations to the texture of enamel and porcelain, and to a certain 

 cryptocrystalliue texture very frequent among volcanic rocks in general, and for which 

 we may apply the obsolete term "lithoid." This passes into the microcrystalline, and 

 always more or less vesicular, texture of trachyte. 



The paste constitutes occasionally alone the substance of the rock. But these 

 instances are rare. More frequently it contains enclosed mineral substances differing 

 from it in nature, and in a few instances these accumulate to such a degree as almost 

 to exclude the paste. Quartz is of the most general occurrence among those which 

 are crystallized. Sanidin is its almost unfailing companion. Oligoclase, usually of a 

 vitreous variety, and black mica, are, too, among the usual ingredients, while horn- 

 blende is generally less conspicuous. Besides these minerals, there are two substances 

 entering accidentally into the composition of rhyolite, which are, however, among the 

 characteristic features of the rocks of this order. One of them consists in small glob- 

 ular grains, from the size of a pin-head to that of a rifle bullet, called " sphaerolites " 

 by Beudant. They have a radial structure, and contain ordinarily a small crystal of 

 feldspar in the center. Certain hyaline, and, in a greater measure, lithoid varieties 

 of rhyolite contain them in large quantity. They occur also, though less frequently, 

 in other natural glasses not of rhyolitic composition, and may be produced artificially, 

 by allowing molten glass to cool very slowly to what is known by the term " Reau- 

 mur's porcelain." The second formation frequently met with in rhyolitic rocks are 

 the "lithophysae," consisting in larger and smaller cavities filled by a substance 

 strangely inflated by some gaseous evolution which apparently originated in the mat- 

 ter itself (Richthofen, 1. c.). They constitute sometimes nearly the entire mass of 

 the rock. 



The endless varieties of rhyolite appear to be due to the susceptibility of the fluid 

 mass to be influenced by accidental circumstances to which it may have been exposed, 

 partly before being ejected, and partly during the process of solidifying. A consid- 

 erable influence, which, however, has not yet been investigated, is probably exercised 

 by the difference in the amount of water which entered into the composition of the 

 molten mass, and partly expanded to steam in the instant of ejection. The vesicular 

 inflation proper to trachytic texture, the spongy inflation of pumice-stone, and the 

 concentric separation of infinitely fine laminae, as is often shown in perfect pearlitic 

 texture, are probably three different modes of manifestation of one slightly varied 

 cause, which may most likely be found in the conversion of water into steam, which 

 participated in the composition of the molten mass. 



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