Apart from organic contents, the mechanically formed deposits of rivers and lakes resemble in general characters the shallow-water deposits of the ocean, though they are usually less widely distributed. It is the accumulations which have actually been formed as æolian rocks, or those which have been laid down as chemical precipitates in salt-lakes which, by study of lithological characters, furnish the most convincing evidence of their terrestrial origin. Many æolian accumulations may be looked upon as soils, if the term soil be used in a special sense to refer to the accumulations which are produced as the result of the excess of disintegration over transportation in an area, whilst others are due to transport which has not been sufficiently effective to carry the material to the sea. When the weathered material accumulates above the weathered rock, it depends chiefly upon climate whether the disintegrated rock becomes mingled with much decayed organic matter forming humus. If this organic matter exists in quantity, the probability is that the accumulation is a terrestrial one, though this is by no means necessarily the case, for under exceptional circumstances a good deal of humus may be deposited in the sea, as beneath the mangrove-swamps which line the coasts of some regions, and to go further back, in the case of the Cromer Forest series of Pliocene times, or some coals, such as the Wigan Cannel Coal of the Carboniferous strata. In addition to the work of water, which affects both land and sea-deposits, the land is especially characterised by the operations of wind and frost upon it, for these produce results which may frequently serve to differentiate a land-accumulation from a deposit laid down beneath sea-level. The effect of wind in rounding the grains of sand which are blown by it is well-known, and samples of the 'millet-seed' sands of desert regions are preserved in most museums. The greater rounding which characterises wind-borne as compared with water-borne sand grains is due, in great measure, to the greater friction between the grains when carried by the air than when swept along by the water. Under favourable circumstances water-worn grains may become rounded, especially when agitated by gentle currents sweeping over a shoal[32]; but a large mass of sand, in which most of the grains have undergone much rounding so as to give rise to 'millet-seed' sand, will nevertheless be probably formed by wind-action except where a marine deposit is formed of material largely derived from an earlier æolian one. The effect of frost is to split rocks into fragments which are more or less angular before they are subjected to water-action. The broken fragments are prone to collect on slopes as screes, and as any scree-material falling into the sea is likely to become rounded except under conditions which rarely prevail, the existence of much scree-material in a rock suggests its terrestrial origin. Glaciers gave rise to terrestrial