R CK-S TR UCT URE. 



427 



it may appear to sight, a compound of two or more elements ? " He 

 takes various specimens of quartz, some perhaps from the granite, others 

 from some other rocks, and subjects them to the analytical processes 

 of the laboratory : the result is, that he finds all quartz, no matter 

 what its color may be, whether white or pink or black, or pure and 

 colorless as glass, to be a compound of the metalloid silicon and the 

 gas oxygen ; in other words, that it is an oxide of silicon, to which 

 he assigns the name silica. By a series of analyses he is able to cor- 

 relate the quartz of the granite with all other forms, and they are 

 many in which this mineral occurs. The flint of the chalk, the white 



5Rm 



Fig. 1. Section of Granite from Cornwall (polarized), magnified 2G diameters. 



veins so often met with in the older slaty rocks, the agates picked up 

 on the sea-shore and elsewhere, the beautiful crystals known as cairn- 

 gorms, amethysts, and others, are all found to be but varying forms 

 of the same substance, colored sometimes by adventitious matter, as 

 iron, etc. ; and he finds, too, that the exquisite skeletons of some of 

 the sponges, the delicate valves of the Diatomacece and other minute 

 specimens of organic life, consist of this very same silica, which is 

 indeed one of the most important compounds entering into the struct- 

 ure of the earth's crust. Suppose the student next picks out one of 

 the feldspar-crystals : this on analysis will be, as was the quartz, found 

 to be also a combination ; in it he will also find silica, but the silica 

 in this instance is found to be combined with the metals aluminium 

 and potassium in fact, is a double silicate of alumina and potash. 

 There are many varieties'of feldspar: some of them differ from that 

 most common in granite, which is called plagioclase, in containing 

 lime or soda instead of potash ; these are also distinguished from the 

 orthoclastic series by their crystalline structure, which will afford, as 

 we shall see, a ready method for their recognition, when they are 



