422 GEOLOGY. 



Regolith.— The first great product of the surface changes is mantle- 

 rock (regolith), which comprehends all the loose matter that springs 

 from rock decay, wear, fracture, and other forms of disintegratioi^. It 

 lies in an unconsohdated sheet on the face of the land, whether as.^oil, 

 sand, clay, earth, gravel, or loose rock. ^" 



Disrupted products: arkose and wacke.— In dry regions, in told 

 regions, on mountain heights and precipitous slopes, and under other 

 conditions where sudden changes of temperature and frost action work 

 efficiently, rocks are broken down into fine fragments without much 

 chemical decomposition. Such disaggregated rather than decomposed 

 matter, if derived from granitic and similar crystaUine rocks, is termed 

 arkose, or arkose sand, and consists of fragments of quartz, feldspar, 

 mica, etc. Common sand consists essentially of quartz grains. If the 

 fine fragments are derived from the darker igneous rocks, and consist 

 mainly of grains of plagioclase feldspar, and ferromagnesian minerals, 

 it is sometimes called wacke. This term is not widely used in just this 

 sense, but there seems to be an important place for it, and it will be so 

 employed in this work. These disaggregated sands are but special 

 phases of the mantle-rock. 



Disintegrated products. — When the surface-rock is chemically decom- 

 posed, the residual material is confined mainly to the insoluble portions, 

 i.e., the siUcious and clayey parts; while the lime, magnesia, soda, 

 potash, and similar substances are largely dissolved and borne to the 

 ocean. The potash is somewhat more disposed to remain with the clays 

 than the soda, lime, or magnesia; but residues of all are usually present. 



Classes of Sedimentary Rocks. 



Shales, sandstones, and conglomerates. — As already shown in the 

 discussion of the atmosphere and surface-waters, the mantle-rock is 

 constantly being borne away and redeposited in lodgment spots on 

 the land or in the basins of the sea, while it is constantly being renewed 

 below. It is an evanescent but ever-renewed derivative mantle. In 

 this process of renewal, removal, and redeposition, the mantle mate- 

 rial is^usually assorted into mud, sand, and gravel; and these several 

 classes of material are laid down more or less separately, and usually 

 take the stratified form, because their deposition depends on different 

 degrees of motion of the transporting waters or wind. When these 



