52 LITHOLOGICAL GEOLOGY. 



and, including the limestones, 50 per cent, of all rocks. Silicon has 

 therefore the same prominent place in the mineral kingdom as 

 carbon in the organic. 



Granite and gneiss are nearly three-fourths silica, — half of it as pure quartz, 

 and the rest as silicates ; mica schist and roofing-slate are about two-thirds 

 silica; trap and lavas are one-half; porphyry, two-thirds; sandstones are 

 sometimes all silica, and usually at least four-fifths. 



Silica is especially adapted for this eminent place among the 

 architectural materials of the globe by its great hardness, its inso- 

 lubility and resistance to chemical and atmospheric agents, and 

 its infusibility. As it withstands better than other common mine- 

 rals the wear of the waves or streams, besides being very abundant, 

 it is the prevailing constituent of sands, and of the movable mate- 

 rial of the earth's surface, as well as of many stratified rocks ; for 

 the other ingredients are worn out by the quartz under the con- 

 stant trituration. It is also fitted for its prominent place by its 

 readiness in forming siliceous compounds and the durability of 

 these silicates. Moreover, although infusible and insoluble, many 

 oxyds enable heat to melt it down and form glass ; or, if but a trace 

 of alkali be contained in waters, those waters, if heated, have the 

 power of dissolving it ; and, thus dissolved, it may be spread 

 widely, either to enter into new combinations, or to fill with quartz 

 fissures and cavities among the rocks, thereby making veins and 

 acting as a general cement and solidifier. 



Its applications in world-making are, therefore, exceedingly 

 various. In all, its action is to make stable and solid. 



(3.) Aluminium. — Aluminium is a white metal, between tin and 

 iron in many of its qualities, but as light as chalk. Combined with 

 oxygen it forms alumina (A1 2 3 ), the basis of clay. This alumina 

 is the gem sapphire, which is next in hardness to the diamond, 

 and of extreme infusibility and insolubility. Alumina is the most 

 common base in the silicates, thereby contributing to a large part 

 of all siliceous minerals, and therefore of all rocks. With quartz 

 these compounds (aluminous silicates) make granite, gneiss, mica 

 schist, syenite, and some sandstones, and alone they form porphyry 

 and other igneous rocks. Nearly all the rocks, except limestones 

 and some sandstones, are literally ore-beds of the metal aluminium. 



(4.) Magnesium. — This metal combined with oxygen forms mag- 

 nesia (MgO), a very refractory and insoluble base, producing with 

 silica a series of durable silicates, very widely distributed : some are 

 quite hard, as hornblende and pyroxene ; others are soft, and have 

 a greasy feel, like talc, soapstone, and serpentine. 



