GROUND WATER 139 



The amount of svich water preserved in the rocks depends, of 

 course, on the volume of the pore space (see below, p. 140), 

 which may be as high as 60% of the volume of the sediment, though 

 of course it is much less in the majority of clastic deposits. 



Lane has called attention to the role played by connate waters in 

 preventing the downward passage of the meteoric waters, which 

 will tend in a measure to dilute some of these stagnant waters. 

 While the meteoric waters circulate freely in the upper zone of 

 the earth's crust (the belt of weathering), the zone of permanent 

 ground water is one of more or less stagnation, and one in which 

 deposition of the mineral matter of these waters in the pores of the 

 rock will take place. We have, therefore, beneath the zone of 

 weathering and free circulation, one of cementation, and thus a 

 barrier to the further downward progress of the meteoric waters 

 is formed. This at the same time forms a barrier against the es- 

 cape of the imprisoned connate waters, which will thus be con- 

 served until erosion of the rocks sets them free once more. Lane 

 was led by such considerations, and by the study of the composition 

 of the deep-seated waters, to conclude that a large part of the 

 ground water attributed by \an Hise and others to a meteoric ori- 

 gin was in reality connate, i. c, the imprisoned water of former 

 oceans. 



General Course of Meteoric Waters. 



Rain falling upon the land is disposed of in several ways, A 

 part of it runs off down the slopes (the run-off), a part sinks into 

 the ground (the absorp), and a part evaporates, returning to the air 

 (the evaporate). The part that sinks into the ground becomes the 

 ground ivater, while the run-off starts the development of surface 

 drainage. Tiie quantitative relation between run-off, evaporate and 

 absorp depends upon (i) the character of the surface, /. e., its 

 topography or slope, porosity of material, state of saturation, 

 amount of vegetable covering, etc., (2) the rate of rainfall or melt- 

 ing of snow, and (3) the subsequent dryness of the atmosphere, 

 and perhaps other local features. \'ery porous soil holding little 

 water will greedily absorb the rain if it falls not too heavily. In 

 this latter case much will run off, as the rate of absorption cannot 

 keep pace with the rate of supply. In a very dry atmosphere much 

 will evaporate before it has time to sink in, while much more will 

 be evaporated from the upper layers of the soil before it has de- 

 scended far, or will be taken up by vegetation to be ultimately re- 

 turned to the air. 



