Page:Memoirs of the Geological Survey of Great Britain, Volume 1.djvu/19

6 influences and the percolation of water, or rather the substances cementing the grains of sand together; for a large proportion of such grains do not readily suffer decomposition, but are again committed to running waters to be again gathered together in banks or beds. Renewed friction may render them somewhat smaller in many cases, but probably, as a whole, after a certain amount of trituration, no very material change is effected in this respect until after a long lapse of time and much grinding against each other, particularly when exposed on shores to the action of heavy breakers. It should be recollected, when considering this subject, that the less the size and weight of the grains, the easier they are mechanically suspended in water, and, therefore, the less are they exposed to the grinding and reducing action to which larger grains or pebbles are subjected when not removed from friction on each other by the same amount of movement in the water.

Not only have we sandstones thus decomposed and removed, but also a variety of slates, marls, and clays, forming matter more readily held in mechanical suspension, as well as hard gravels, either still loose and incoherent, or consolidated as conglomerates. To the former class of substances, easily held in mechanical suspension, we must add the decomposed felspars of a large portion of the igneous rocks, in some countries a matter of much importance chemically and mechanically, the silica and potash being removed by the former method dissolved in the water, while the silicate of alumina is mechanically transported and deposited as clay. This decomposition of the felspar usually disintegrates the rocks of which it forms a part, and quartz, mica, and other minerals, as the case may be, come under the mechanical influence of water, and are moved to new situations by it. Angular fragments are formed in abundance from the effect of atmospheric influences, especially in mountainous regions, and on the higher parts of sea cliffs.

According to the mountainous, hilly, or plain character of a country, is the fall of the drainage depressions and their general character, the ordinary rapidity of the brooks and rivers in them, and consequently the power of mechanically removing and transporting the mineral matter which is either worn away by these running waters, or which may be borne into them from the decomposed rocks adjacent. When the drainage is interrupted by cavities, the waters spreading out into lakes of adequate size, the detritus mechanically borne onward by the waters is arrested, and before a continuous line of running water can be formed the cavity must be filled up. A study of the entrance of running water into lakes soon shows how this is accomplished.

When committed to running water, the substances to be transported mechanically are moved in two ways. Should the particles be small, they are readily caught up in mechanical suspension, and carried for-