The imperceptible slowness with which many of these vast changes are actually taking place warrants the inference that violent volcanic revolutions have no doubt been far less instrumental in moulding the earth-rind to its present form than the slow oscillatory movements of elevation and depression which from time immemorial have been constantly altering its surface. The causes of these oscillatory movements are still very imperfectly known, though a probable hypothesis attributes them to the expansion by increased temperature of extensive deep-seated masses of matter. As the elevation of some tracts seems to coincide with the proportionate depression of others at a greater or less distance, these alternating upheavals and subsidences may possibly be the result rather of the lateral shifting of the flow of heat from one mass of subterranean matter to a neighbouring mass than of its positive increase on the whole. ‘Such a lateral diversion of the outward flow of heat,’ says Mr. Poulett Scrope, ‘we may presume to be caused by the deposition over certain areas of thick newly-formed beds of any matter imperfectly conducting heat, like sedimentary sands, gravels, clays, shales, or calcareous mud, by which the outward transmission of heat being checked, it must accumulate beneath, while a portion of it will pass off laterally to augment the temperature of mineral matter in neighbouring areas; just as the water of a spring, if its usual issue is blocked up, will accumulate in the fissures or pores of the rock containing it, until it finds a vent on either side and at a higher level. Owing to this increase, the resistance opposed by the overlying rocks in that quarter may be sooner or later overcome, and their elevation brought about, through the dilatation of the mineral matter beneath.’ CHAPTER V. SUBTERRANEAN WATERS AND ARTESIAN WELLS. Subterranean Distribution of the Waters—Admirable Provisions of Nature— Hydrostatic Laws regulating the Flow of Springs—Thermal Springs— Intermittent Springs—The Geysir—Bunsen’s Theory—Artesian Wells—Le Puits de Grenelle—Deep Borings—Various Uses of Artesian Wells—Artesian Wells in Venice and in the Desert of Sahara. In every zone the evaporating power of the sun raises from the surface of the ocean vapours, which hover in the air until, condensed by cold, they descend in rain upon the earth. Here part of them are soon restored to the sea by the swollen rivers; another part is once more volatilised; but by far the larger quantity finds its slow way into the bowels of the earth, where it serves for the perennial supply of wells and springs.