GEOGNOSY AND GEOLOGY. 91 
both in homogeneous and heterogeneous rocks. In amorphism we 
distinguished the earthy, opaline, and vitreous. Genetically considered, 
the opaline bodies appear to have been produced by the gradual 
solidification of a gelatinous matter; the vitreous, by the rapid cooling of a 
melted mass. 
Order 1st. Silicious Rocks. 
This order embraces the rocks in which silex forms the principal 
ingredient : we may divide it into three sections. 
Section 1. Quartz Rocks ; including: 
Quartz rock proper ; with its modifications, the compact, the granular, 
and the slaty quartz. Compact quartz rock is of a splintery fracture; the 
granular approximates closely to quartz sandstone, in fact it is difficult to 
draw the line of distinction. Here also belongs the silicious frit, which is 
nothing else than an incomplete melting together, or agglutination of quartz 
sand. It occurs in the vicinity of volcanic masses, to whose influence it 
owes its origin. The slaty texture of the last modification, the slaty quartz 
rock, is to be ascribed to mica, whose crystalline lamine, lying in the same 
plane successively, thus permit a separation. The remarkable part 
performed by the mica, in this way, will be referred to more fully hereafter, 
many rocks deriving from it the property of splitting in definite directions. 
When quartz rock, which is generally of a light color, acquires a darker 
tint by the addition of oxyde of iron and manganese, aluminous matter, 
&c., it passes into: 
Argillaceous quartz rock, to which also belongs stlicious slate, a 
combination of crystalline and amorphous silex, with clay slate. This is 
distinguished into a common, with splintery, and a jaspery, with conchoidal 
fracture. Its color is generally black, although sometimes occurring grey, 
green, or brown. It frequently contains anthracite, and it is to carbon in 
this form that silicious slate probably owes its black color. Silex traverses 
it in veins, always of .a white color, the carbon of the rock never 
penetrating these veins. Calcareous spar also occurs in veins in a similar 
manner. 
Jasper. This is an intimate combination of silex with a little alumina : 
generally colored brown by iron. It rarely occupies an extended place 
among rocks, being quite restricted in its occurrence. It sometimes 
incloses crystals of feldspar, in which case it becomes porphyritic. It 
passes into whetstone, and silicious shale. It has frequently a strong 
resemblance to a burnt clay in the so called porcelain jasper, a clay baked 
by igneous action. The banded jasper is a variety exhibiting layers of 
different color. 
Section 2. Hornstone. 
The rocks belonging under this head consist of an intimate combination 
of quartz and compact feldspar. The principal of these is the hornstone 
rock, whose most remarkable transitions are into quartz rock. and a 
compact feldspar rock called whitestone. An increase in the quantity of 
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