Comparison of Residual and Transported Clays.—§ 23. We can now understand the differences between such residual and transported clays as have been described above. These residual clays originate through the decomposition of crystalline rocks. They can only be pure or of high grade when the rocks from which they are derived are made up en tirely of minerals which contain only silicates of alumina and of other bases whose salts formed with the acids of ground-water are soluble, and then only when the movement of the water is free enough to carry away these soluble salts as fast as they are produced. These conditions are almost never met. The purest deposits usually contain crystals of quartz and of other minerals which are not readily attacked by the acids of ground-water, and these are removed before such clays are used by weath Bring the clay or by washing, usually both. Granitoid rocks usually con tain a considerable percentage of minerals which carry iron, magnesia, lime and other bases, metallic and non-metalic, in such quantity that they do not form soluble compounds when the rocks decompose, and are re tained in the earthy residual mass. When the movement of the water is in any way obstructed, even the soluble salts tend to reunite with silica and precipitate in the form of zeolites and similar minerals, or are de posited direct as carbonates or salts of other acids. For these reasons most residual clays are too impure for use in the manufacture of high grade ware.
When the residual material is carried away by running water it is usually assorted before redeposition. If it were not for changing velocity of currents all the coarser decomposed material would be laid down as separate beds of sand and gravel, and all dissolved matter would be car ried on to the sea or find place in some special mineral deposit, leaving the clay by itself in relatively pure masses. This assortment of material actually does occur occasionally, and very pure deposits of clay are some times formed in this way. It always occurs in a greater or less degree, but we must not infer from this that transported clays are always more nearly pure than those formed in situ. They often are, but during their journeys they are exposed to many sources of contamination. Rivers erode their beds and rains wash materials of various kinds, organic and inorganic, into the current, so that it often happens that the transported clays are much less pure than the beds from which they were derived.
Transportation and Final Deposition of 24. All such deposits of transported clays as have been described are to be re garded as temporary only. They will in turn be broken down and car ried further. Whenever the debris of granitoid rocks is picked up by running water it starts on a journey whose only end is the sea. This is as true of that which is carried in solution as of that in suspension. Both may find temporary lodgment many times on the way ; the dissolved substances in the form of ores and other mineral. deposits, the suspended substances in that of earthy masses, but all of these will later be taken up again and continue their journey. Every particle which enters into the composition of the original rock will in time find its way to the sea. Arrived at the sea the suspended materials are deposited on its shallow margin in beds roughly parallel to the shore line. Here again the debris is more or less perfectly assorted into beds of gravel, sand or clay, and outside the clays in the clear water, sea animals absorb the dissolved lime and make of it skeletons and shells, which upon the death of the animals are ground into lime-sand and eventually consolidated into limestones.
Certain animals too prefer to live in muddy or sandy water. These also have the power to absorb lime from water and with it to harden their tissues. Upon their death their hard parts are ground and built into the clays and sands.
Variations in velocity of the inflowing stream, in tidal action, and in storm waves; as well as changes in level on the sea margin, bring with them changes in position of the belts in which the assorted materials are laid down, and so produce alternations of gravel, sands, clays and lime stones. As the beds thicken these layers follow each other without regular sequence and so build up masses of sedimentary material, which sometimes reach a thickness of thousands or even tens of thousands of feet. In the central part of the Appalahcian region such sediments are thought to have accumulated to the depth of 60,000 feet.
Formation of 25. In such masses enormous pressures are generated, and the water with which they are saturated is rich in dis solved materials and moves but slowly; consequently in part by precipi tation of dissolved material which acts as a cement, in part by recrystal lization of amorphous (colloidal) matter, and in part by pressure bring ing contiguous surfaces so near that they are held by molecular attrac tion, the loose sediments are consolidated into conglomerates, sandstones. shales and limestones. The alternation of coarse and fine particles due to slight variations in the currents during deposition, together with this pressure, develops a shell-like, shalt' structure in this mass of clay, roughly parallel to the bedding, and partly on account of the recrystal lization of the fine particles of clay, partly because of the cementation of its grains by dissolved lime, iron or silica, or partly because of the simple cohesion of the plates and grains, the plasticity of the clay is lost, but it may be regained by sufficiently fine grinding which restores the material to its condition.
In § 26 we have shown how changes at the seashore, while deposits were being laid down, cause a layer of clay to be over or underlaid by one of sandstone or limestone. Changes smaller than these or of shorter duration may incorporate grains of sand with the deposit of clay and so make it more or less siliceous. On the other hand, the tests (hard covering) of silica-secreting animals and plants may be de posited in considerable amounts with the clays, and silica, which is always present in sea water, may be deposited in the inter-granular spaces of the solidfying mass. In one or more of these ways the clays may become very siliceous : in fact there is a regular gradation between pure clay on the one hand and pure sand on the other.
In the same way lime-secreting animals may live upon the bottom, and their hard parts be ground and mixed with the mud. Lime is also often chemically precipitated between the grains of the hardening mass, and so we have a regular gradation between pure limestone on the one hand and pure clay on the other. Iron, too, may be incorporated in the same way, and so when these clays are compacted into shales we may have calcareous, siliceous or ferruginous shales when these adulterants are less in amount than the clay, or argillaceous limestones, argillaceous sandstones, etc., when they are in greater amount. Shales then may be composed of absolutely pure clay or of clay mixed with lime, iron, silica, or any other substances deposited by sea water from suspension or solution, and this mixture may occur in any proportions. This is so true that it is a rare thing to find either limestone, shale, or sandstone which does not contain appreciable amounts of the other ingredients.