n6 THE BOOK OF NATURE STUDY 



Evidently to make such a rock at least one and usually more 

 kinds of solid stone must have been broken to pieces, the bits 

 rounded, and then fastened together. We have already seen 

 that denuding processes are capable of effecting the first two 

 results. 



Sandstone is merely a conglomerate on a small scale. It is 

 made of fragments like grains of sand, broken up and rounded, often 

 stained and coloured, and then cemented together, sometimes 

 loosely, but often compacted into hard stone. The chief difference 

 between sandstone and conglomerate, apart from the scale of the 

 material, is that the fragments, instead of being bits of recognis- 

 able rocks, are now bits of broken and rounded minerals. The 

 chief constituents are rounded grains of a clear, transparent sub- 

 stance, which chemical and microscopical examination proves to 

 consist of that form of crystalline silica called quartz. Other 

 broken minerals are present in smaller quantities, such as felspar, 

 mica, and iron oxides. These constituents are mixed with mud 

 usually, and the whole cemented by carbonate of lime, silica, or 

 oxide of iron which gives the rock its colour. 



It will be seen later that the disintegration of crystalline 

 rocks would yield minerals which might be broken up into the 

 constituents present in sandstone ; and it has already been shown 

 that ordinary denuding processes result in the production of great 

 quantities of sand, which may be either river-borne or deposited 

 on the seashore and below sea level. This sand has precisely the 

 same chemical and mineralogical character as the sand of sand- 

 stone (Fig. 21), and the two are identical in every respect, even 

 to the amount of rounding the grains have undergone and the 

 staining of them by iron oxide. A similar rock, but made of 

 angular quartz and felspar grains, is known as a grit, and it bears 

 the same relation to sandstone that breccia does to conglomerate. 



Finer grained rocks are called clay if soft, shale if slightly 

 hardened and laminated, slate if much indurated. All agree in 

 the fact that they consist of very fine particles of hydrated 

 silicate of alumina (china-clay or kaolin) reduced to a fine powder, 

 and mingled with grit particles very minute in size. Cement may 

 be present as before, but it is usually not carbonate of lime, nor 

 is it conspicuous in quantity, the hardening being mostly effected 



