The mere distance of hills or mountains need not discourage us from making trials; for the waters which fall on these higher lands readily penetrate to great depths through highly-inclined or vertical strata, or through the fissures of shattered rocks; and after flowing for a great distance, must often reascend and be brought up again by other fissures, so as to approach the surface in the lower country. Here they may be concealed beneath a covering of undisturbed horizontal beds, which it may be necessary to pierce in order to reach them. The course of water flowing underground is not strictly analogous to that of rivers on the surface, there being, in the one case, a constant descent from a higher to a lower level from the source of the stream to the sea; whereas, in the other, the water may at one time sink far below the level of the ocean, and afterwards rise again high above it. For the purposes under consideration, we may range the various strata of which the outer crust of the earth is composed under four heads, namely: 1, drift; 2, alluvion; 3, the tertiary and secondary beds, composed of loose, arenaceous and permeable strata, impervious, argillaceous and marly strata, and thick strata of compact rock, more or less broken up by fissures, as the Norwich red and coralline crag, the Molasse sandstones, the Bagshot sands, the London clay, and the Woolwich beds, in the tertiary division; and the chalk, chalk marl, gault, the greensands, the Wealden clay, and the Hastings sand; the oolites, the has, the Rhætic beds, and Keuper, and the new red sandstone, in the secondary division; and 4, the primary beds, as the magnesian limestone, the lower red sand, and the coal measures, which consist mainly of alternating beds of sandstones and shales with coal. The first of these divisions, the drift, consisting mainly of sand and gravel, having been formed by the action of flowing water, is very irregular in thickness, and exists frequently in detached masses. This irregularity is due to the inequalities of the surface at the period when the drift was brought down. Hollows then existing would often be filled up, while either none was deposited on level surfaces, or, if deposited, was subsequently removed by denudation. Hence we cannot infer when boring through deposits of this character that the same, or nearly the same, thickness will be found at even a few yards' distance. In valleys this deposit may exist to a great depth, the slopes of hills are frequently covered with drift, which has either been arrested by the elevated surface or brought down from the upper portions of that surface by the action of rain. In the former case the deposits will probably consist of gravel, and in the latter, of the same elements as the hill itself.