1. Almost all the minerals which occur in the geological formations are, to some slight extent, _soluble in water_. Hence, rain water, by passing through a stratum of earth or rock and reäppearing as a spring, loses the insipidity which it had as pure water, and becomes palatable. It is then found to hold in solution some small proportion of earthy substances, upon which this change of taste depends. Although the proportion of dissolved matter is very small, yet the surface of earth upon which this distilled water is shed is one-fourth of the surface of the globe, and solution below all that surface is constantly taking place. No inconsiderable amount must thus have been furnished, from the existing rocks of each period, towards the formation of the strata of a later period. There are some substances which are soluble in water, in large quantities. _Rock-salt_ is an example. It is not found in any very large proportion in rocks generally, but a very large aggregate amount has been taken up by the waters which have filtered through the strata. The ocean gathers into itself, by degrees, all the soluble substances which are thus taken up. It receives supplies of water charged with these substances from springs, rivers and lakes. It returns as much water as it receives; but it is always in the form of vapor, and is therefore pure water. Hence the saline properties of the ocean, and of those inland seas which have no outlets. There is thus gathered the materials for the rock-salt deposits. But many substances which are not considered soluble in water become so by some modification of the water. Water of a high temperature is capable of dissolving silex. In Iceland and other volcanic regions, the hot springs are charged with silex, which is deposited as the water cools. Thus, siliceous formations accumulate around springs of this kind. The various agates may have been deposited from such solutions. In the decomposition of mica, felspar and volcanic rocks, a considerable amount of potassa is set free. Potassa or soda renders the water in which it is dissolved capable of dissolving silex in large quantity. In these ways water removes, with some degree of rapidity, one of the most insoluble minerals which rocks contain.