14 RICHTHOFEN—NATURAL SYSTEM 
difference in character from that which is a modification of other voleanic 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 
cryptocrystalline 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 “ Réau- 
mur’s porcelain.” The second formation frequently met with in rhyolitic rocks are 
the ‘“‘lithophyse,” 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 lamin, as is often shown in perfect pearlitic 
texture, are probably three different modes of manifestation of one slightly varied 
eause, which may most likely be found in the conversion of water into steam, which 
participated in the composition of the molten mass. 
(52) 
