The permeability of such beds will, of course, depend wholly upon the nature of the deposit. Some rocks produce deposits through which water percolates readily, while others allow a passage only through such fissures as may exist. Sand and gravel constitute an extremely absorbent medium, while an argillaceous deposit may be wholly impervious. In mountainous districts springs may often be found in the drift; their existence in such formations will, however, depend upon the position and character of the rock strata; thus, if the drift cover an elevated and extensive slope of a nature similar to that of the rocks by which it is formed, springs due to infiltration through this covering will certainly exist near the foot of the slope. Upon the opposite slope, the small spaces which exist between the different beds of rock receive these infiltrations directly, and serve to completely drain the deposit which, in the former case, is, on the contrary, saturated with water. If, however, the foliations or the joints of the rocks afford no issue to the water, whether such a circumstance be due to the character of their formation, or to the stopping up of the issues by the drift itself, these results will not be produced. It will be obvious how, in this way, by passing under a mass of drift the water descending from the top of hill slopes reappears at their foot in the form of springs. If now we suppose these issues stopped, or covered by an impervious stratum of great thickness, and this stratum pierced by a boring, the water will ascend through this new outlet to a level above that of its original issue, in virtue of the head of water measured from the points at which the infiltration takes place to the point in which it is struck by the boring.