CIRCULATION LARGELY SHALLOW. 589 



GENERAL STATEMENT. 



In conclusion of this part of the subject, while the lessened viscosity 

 with depth, slow movement, and long journey are all very favorable to a 

 deep circulation, I have little doubt that in porous strata large openings 

 near the surface and the more direct course of a shallow journey result, on 

 the average, in much larger flowage in the upper part of the belt of cemen- 

 tation than in the lower part. 



TEMPERATURE OF ENTERING AND ISSUING WATER, AND TRANSFER OF HEAT. 



Before taking up the work of ground water in the belt of cementation 

 it is necessary to consider the relative temperatures of the water which 

 enters and departs from the sea of underground water. Is the water which 

 joins the belt of cementation colder or warmer than that which issues 

 from it! 



At a certain definite distance below the surface, in most regions from 

 12 to 15 meters, seasonal variations in temperature are eliminated; the 

 thermometer discloses no difference between the temperature of winter 

 and that of summer. The thermal conductivity of the rocks is so low and 

 the amount of water which passes to the belt of cementation is relatively 

 so small that the g*reat variations of temperature of the atmosphere, of 

 the rocks at the surface, and of the meteoric water, emphasized on pages 

 433-444, 458-460, at the depth of a few meters all merge into a temper- 

 ature uniform for a given locality, but of course variable in different 

 localities, and especially with latitude. 



When the rain falls upon the soil it may be warmer or colder than the 

 surface of the earth, but the part that sinks into the soil is subdivided 

 between the pores of the rocks and comes into most intimate contact 

 with them, so that when the water at the surface is warmer than the rocks 

 it is cooled; when colder than the rocks it is warmed. Thus the two 

 quickly assume nearly the same temperature. Doubtless downward-moving 

 water is one of the most important of the agents which determine the fixed 

 average annual temperature of the rocks a short distance below the surface. 

 The reason for this belief is the high specific heat of water as compared 

 with rocks. However, the relative influence of the heat given by water 

 and that due to other climatic factors, such as latitude, altitude, etc., need 



