GRANITE FAMILY 1 97 



Differences of texture, produced in the manner already described, 

 give rise to rocks of totally different appearance, which it is diffi- 

 cult to imagine are of similar or identical composition. 



Obsidian is a volcanic glass, which is usually black or dark 

 brown or green (but sometimes blue, red, or yellow). It breaks 

 with a shell-like fracture, and in very thin pieces is translucent. 

 The microscope shows that its dark colour and opacity are due to 

 the quantity of minute " crystallites," the incipient stages of crys- 

 tals, which are present in great numbers. The name obsidian is 

 used for the various kinds of volcanic glass in which the percent- 

 age of water is small, and so for exact description a prefix is 

 necessary, such as rhyolite obsidian, andesite obsidian. Though 

 the glasses are of varying composition, by far the greater number 

 of them belong to the granite family. When the glass is divided 

 by concentric cracks, due to shrinkage on cooling, so as to form 

 onion-like spherules, it is called Perlite. 



Pitchstone. has much the same appearance as obsidian, but 

 contains from 5 to 10% of water. 



Pumice is a glass blown up by the bubbles of escaping steam 

 and other vapours into a rock froth, so light that it will float upon 

 water. A very similar substance is produced when a jet of steam 

 is blown through the melted slag from an iron furnace. 



It not infrequently happens that, in course of time, the volcanic 

 rocks become devitrified, losing their glassy texture and assuming 

 a stony one. The homogeneous rock becomes converted into a 

 mass of extremely minute crystals of quartz and felspar, and 

 the original glassy texture is then shown only by the lines of 

 flow, or by the perlitic Character, which are not affected by the 

 change. 



Rhyolite ordinarily occurs as the lava outflow of a granitic 

 magma, cooled rapidly, but yet more slowly than obsidian. The 

 texture is porphyritic, the phenocrysts being chiefly quartz, and 

 the glassy form of orthoclase known as sanidine, while the ferro- 

 magnesian minerals are present in very much smaller quantities, 

 and of these the commonest is biotite. The phenocrysts are 

 embedded in a ground mass of minute felspar crystals and a 



