Page:Memoirs of the Geological Survey of Great Britain, Volume 1.djvu/18

Rh In a similar manner, other substances are carried from situations where they have remained fixed from early geological times to form solid matter in new localities, often combined in a different manner, but still being merely again used, as it were, in keeping up a general mass of solid mineral matter on the surface of the globe.

In all decompositions of felspar or albite, so abundant in granitic and many trappean and volcanic rocks, by carbonic acid, the silica in its soluble state from silicate of potash or of soda, as the case may be, is carried away by water. This silica, when deposited in cracks, forms quartz, and compounds with other substances, or cements sands together, forming the sandstones of future times. Independently of siliceous deposits, as from the Geisers of Iceland, it is appropriated even by marine animals, such as sponges and some molluscs, for parts of their structure essential to their well-being.

The soluble matter thus carried away by waters percolating through rocks should receive careful attention. The cementing substances of sandstones is frequently thus gradually removed and borne away chemically, while the grains of sand having no coherence among themselves in the situations longest exposed to this disintegrating action are carried off mechanically, the grains of sand and their cementing matter again forming solid rock under new conditions, though probably seldom again united in the same mass.

Atmospheric influences and the descent of rain water into rocks, even to depths where the water acquires a heat rendering it capable of effecting a solution of substances which could not have happened at comparatively low temperatures, thus cause the passage of soluble mineral matter from one place to another, cracks and other cavities being filled with different compounds, bearing various names, and becoming under favourable conditions crystallized. Hard rocks are decomposed in one place, and the matter which they have lost consolidates beds of loose materials in another, so that we may consider the compound parts of a large amount of rocks as silently and slowly moving from one place to another during the lapse of time, new dislocations of the surface of the earth, and the protrusion of igneous products of various kinds through that surface greatly aiding in the chemical change to which so many masses of rock are subjected.

Very important modifications of rocks must be effected by any covering of melted rocks flowing over them, particularly when such rocks are permeated by moisture; for then we have heat, water, and often the vapours of various kinds, the fruitful causes of many chemical changes, acting on them, sometimes giving to the rocks greater solidity, at others aiding their decomposition and disintegration.

We have seen that sandstones become decomposed from atmospheric