50 LITHOLOGICAL GEOLOGY. 



constituent of sands, and of the movable material of the earth's sur- 

 face, as well as of many stratified rocks ; for the other ingredients are 

 worn to the finest powder by the quartz, under the constant trituration, 

 so as to be drifted away by the lightest currents. 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 alone, when mixed with different oxyds it melts and 

 forms 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 any fissures and cavities among the rocks, thereby 

 making veins and acting as a general cement and solidifier. 



Its applications in world-making are 5 therefore, exceedingly various. 

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



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

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

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

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

 and of extreme infusibility and insolubility. It 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 granyte, gneiss, mica schist, syenyte, and 

 some sandstones, and alone they form trachyte and some other igneous 

 rocks. Nearly all the rocks, except limestones and many sandstones, 

 are literally ore-beds of the metal aluminum. 



(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. 



Unlike alumina, magnesia unites with carbonic acid, forming car- 

 bonate of magnesia (MgO, CO 2 ). 



(5.) Calcium. — The oxyd of the metal calcium is common quick- 

 lime. Like magnesia, it enters into various silicates ; and it also 

 forms a carbonate, carbonate of lime (CaO,C0 2 ), and this carbonate 

 is the material of limestones. Moreover, with sulphuric acid and 

 water, it forms sulphate of lime, or gypsum. 



The peculiar position of lime in the system of nature is that of a 

 medium between the organic and inorganic world. Carbonate of lime 

 is soluble in water, when a little carbonic acid is present in solution ; 

 and both this and the sulphate are found in river, marine, and well 

 waters. It is made into shells, corals, and partly into bone, by ani- 

 mals, and then turned over to the inorganic world to make rocks. 



