50 ORIGIN OF STRATA. 



called grains, which, ordinary water does not easily dissolve; and 

 when the granite becomes softened by exposure to water containing 

 carbonic acid, these particles, which are relatively heavy, are washed 

 out of the rock by rain or the waves so as to form grains of sand. 

 Because this is the heaviest material derived from the wasting of 

 granite, it is carried to a less distance from the shore than other 

 substances, and forms a belt of sand or sandstone parallel to the coast. 

 If granite were the only source of water-formed rocks, and only 

 denuded by the sea, one quarter of all known stratified formations 

 would be arenaceous or sandy deposits. And if the mineral mica, 

 which is a glistening flaky substance, is supposed not to be altered in 

 character or decomposed, it will often be carried to the limit of the 

 sand and help to form what is called a micaceous sandstone, causing 

 the rock afterwards to split into thin layers. But sometimes the 

 mica is carried much farther. More than half of the granite consists 

 of felspar, which forms large milky-white or red crystals. As we 

 have already mentioned, this mineral is chiefly composed of a silicate 

 of alumina ; but in granite it also includes some potash, soda, a little 

 lime, iron, and other substances. The carbonic acid, which is always 

 dissolved in water, attacks the felspar by dissolving out from it car- 

 bonates of potash, soda, or lime; and then the crystals lose their 

 hardness and become changed into a paste of impalpable fine particles 

 which forms a mud. This mud is held in suspension longer than the 

 sand, and is therefore carried farther out to sea. When it falls to 

 the bottom and is compressed by the weight of water above, it becomes 

 clay, and margins or surrounds the land as an outer belt probably 

 twice as broad as the sand belt. On some coasts, like the South 

 American coast mentioned by Mr. Darwin, there may be no clay 

 deposited within one hundred and fifty miles of land. There is no 

 clearly defined separation between the limits of clay and sand, for 

 they pass into each other from the materials being mixed, and the 

 clays nearest to shore are often micaceous. 



When this denudation takes place upon land by the agency of 

 rain, as on Dartmoor, which consists of granite, the sand is left on 

 the slopes of the mountains, while the mud is carried down into the 

 valleys, where it forms pipe-clay. Usually coast and surface de- 

 nudation go on together, and much of the surface mud is carried 

 away by the rivers ; so that the result on the sea-bed is, that as the 

 clay-deposit which was derived from shore denudation approaches 

 a river mouth, the mud which the river brings down causes il 

 to extend out to sea in an expanded fan-like form. So that whil 

 the coast clay has its greatest extension in the line of the coast, il 

 follows that the river clay has its greatest extension in the line of tl 

 river, which is usually at right angles to the coast. The river 

 is also much thicker than the shore clay, for it is the mud derive 

 from a large area of land, and hence it sometimes persists unchan< 

 through several geological formations, while the shore-deposits 

 come altered in their mineral materials. There can be no doubt that 

 the rusty colour of many sandstones is due to the decomposition 

 mica after it was deposited, so that the iron was set free as an oxide. 



