The solution of mineral matter by ground water, as well as the other chemical changes it effects, is greatly augmented by the impurities, especially carbonic and other organic gases, dissolved by the water from the atmosphere and the soil. The commonest chemical changes effected by the joint action of water and air, oxidation and carbonation, have been referred to in Chapter II. Hydration is more exclusively the work of water, and is one of the commonest processes of rock change, and often of rock disintegration. Numerous other less simple chemical changes resulting from the activities of ground water are constantly in progress, and in so far as they lead to the disintegration of rock are processes of weathering. Many chemical changes involve notable changes in volume of the mineral matter concerned. Merrill has calculated that in the conversion of the granitic rock of the vicinity of Washington, D. C., into soil, its volume has been increased 88 percent., largely as the result of hydration.[45] Even when the chemical changes do not themselves directly involve the disintegration of the rock, the accompanying increase of volume is sometimes sufficient to cause its physical disruption. This also may be regarded as a phase of weathering. The weathering accomplished by water, or under its influence, proceeds at rates which vary with the composition of the rock, the amount and composition of the water, the temperature, and certain other factors less susceptible of brief statement. The weathering effected by ground water has a wider range both in area and depth than that due to changes of temperature, for while the latter is effective only where temperature changes are considerable, and where coherent material lies at the surface (p. 45), the former is operative to all depths to which water sinks. [Illustration: +Fig.+ 94.—Talus accumulation at the base of a steep bluff. Weber Canyon, Uinta Mountains, Utah. The talus has accumulated since the last glaciation of the valley and is therefore of very recent origin. (Church.)] There are other processes of weathering not due directly either to the atmosphere or to water. The roots of trees and smaller plants frequently grow into cracks of rocks, and, increasing in size, act much like freezing water (p. 45) in similar situations. This wedge-work of roots is a phase of weathering.