214 GEOLOGY, 



the amount of precipitation the greater the amount of water which 

 will sink beneath the surface. Other things being equal, a region of 

 heavy precipitation is a region where wells are easily obtained and 

 springs common. (2) Bate of precipitation. — A given amount of pre- 

 cipitation may be concentrated in a short interval, or distributed through 

 a considerable period of time. In the latter case more of the water 

 sinks beneath the surface; in the former, a larger proportion runs 

 off over the surface. The reason is readily seen. Water passes through 

 small spaces, such as those of soil, slowly, and its rate of passage decreases 

 rapidly with decreasing size of the passage-ways. When rain falls 

 rapidly on a surface of even moderately close texture, the uppermost 

 layer of soil is promptly filled with water, and since the water passes 

 downward slowly, the uppermost saturated part of the soil becomes 

 virtually impervious. While in this condition, the water which falls 

 on it will run off if there be slope, and stand if there be none. In the 

 latter case it will sink slowly as the water in the soil passes down to 

 lower levels. If precipitation takes place no faster than the water 

 can sink through the soil, all the water may become ground-water. 

 (3) The topography of the surface has much to do with determining 

 the proportion of rainfall which becomes ground-water. If the sur- 

 face be flat, more will sink in; if it be sloping, more of it will run off 

 before it has time to sink. Other things being equal, the steeper the 

 slope the larger the proportion of the rainfall which will run off over 

 it. (4) The texture of the soil, or other material on which the rain falls, 

 helps to determine what proportion of it sinks beneath the surface. 

 If the surface materials be porous, the water sinks readily; if of close 

 texture, it finds less ready ingress. Other things being equal, the 

 closer the texture of the soil the less the proportion of the rainfall which 

 will enter it. (5) The texture and structure of the rock beneath the sur- 

 face have some influence on the amount of ground-water. The rock 

 may be stratified or massive; it may be abundantly or sparsely jointed; 

 it may be porous or compact. On the whole, stratified rock is more 

 favorable for the entrance of water than unstratified, partly because 

 of its greater average porosity, and partly because the planes of division 

 between beds often allow the passage of water. If the beds of strati- 

 fied rock are vertical or inclined, water finds its way into them more 

 readily than if they are horizontal, in so far as it descends along strati- 



