HEAT INFLUENCES IN UNDERGROUND CIRCULATION. 149 



In some countries the issuing waters throughout great regions are very 

 clearly at a higher temperature than the entering waters, and in such 

 regions the difference in' temperature must be a very important factor in 

 the underground circulation. In such cases the difference in temperature 

 between descending and ascending waters generally residts from a combina- 

 tion of the normal increase of temperature due to depth, from regional 

 volcanism, and from the rocks having a higher temperature than normal 

 because of recent orogenic movements. An excellent illustration of such 

 regions is the Cordilleran region of western United States. (See pp. 591- 

 592.) 



As already noted, the expansion of water with increase of temperature 

 is considerable, amounting to over 4 per cent between 0° and 100° C; 

 {hat is, a given mass of water occupies a volume 4 per cent greater at the 

 latter than at the former temperature. In other words, if there be an 

 average difference of 100° C. between the ascending and descending 

 columns, 100 meters of the downward-moving water balances 104 meters of. 

 the upward-moving- water. If we suppose the descending and ascending 

 columns to be connected, of equal height, and having an average difference 

 in temperature of 100° C, this would be equivalent to a head of 4 meters per 

 100 meters for the entire height of the column. Probably the difference in 

 temperature between two columns is not often so great as 100° C, but if it 

 be sufficient to give a difference in density of 1 per cent, and the ascending 

 and descending columns be the same length, this is ample to give a stress 

 sufficient to overcome friction and viscosity, and give a decided movement 

 to ground water. As an illustration of the principle may be mentioned the 

 water power of the sea mills of Cephalonia, which, according to the Crosbys, 

 is wholly due to difference in temperature between the descending- and 

 ascending waters." 



Mechanical action. — A third force influencing ground-water circulation is 

 mechanical action. Earth movements may close or partly close the 

 openings in the rocks, and in this process squeeze out the water, as in the 

 production of the schists and gneisses from the sedimentary rocks. If the 

 deformation of the rocks be referred to their ultimate cause, gravity, even 

 the circulation of the water resulting from deformation is indirectly due to 

 the stress of gravity. 



" Crosby, W. F., and Crosby, W. O., The sea mills of Cephalonia: Tech. Quar., vol. 9, 1896, 

 pp. 6-23. 



