148 A TREATISE ON METAMORPHISM. 



cent or more is probably not uncommon. Decrease in temperature may 

 correspondingly increase the density of water. 



Gravity and heat — While change of temperature necessarily involves some 

 movement, its chief effect in water circulation is as a force subordinate to 

 gravity. In so far as water in a connected descending and ascending 

 system is warmer at its point of issuance than it was when it joined the sea 

 of underground water, this gives gi-avity an effect in circulation in the 

 same direction as head. This is consequent upon the fact, noted above, 

 that the density of water varies inversely with the temperature. 



It is therefore evident that in columns of water of equal length the 

 stress of gravity is greater upon the column having the lower temperature. 

 That the diffence in gravitative stress due to difference in temperature may 

 be sufficient to produce rapid circulation in pipes that are supercapillary is 

 shown by the use of the principle in the hot-water system of heating 

 buildings. Underground, as in the hot-water system of heating, heat is the 

 energy which causes the water to expand, and gives a difference in density. 

 When heat has produced a difference in density of the two columns, 

 gravity is the force which inaugurates and maintains the circulation. 



It is believed that underground circulation may be promoted to an 

 important extent by difference in temperature of the descending and 

 ascending columns of water, resulting from heat abstracted from the rocks 

 due wholly to their normal increment of temperature with depth. Later it 

 will be shown that the downward-moving water is ordinarily dispersed in 

 many small openings and moves relatively slowly; therefore it may be 

 supposed at any given place to have approximately the temperature of 

 the rocks. The upward movement of water, on the contrary, is shown to 

 be usually in the larger openings and relatively rapid; therefore at any 

 given place its temperature is probably higher than is normal for the rocks 

 at that depth. The result is a difference in temperature between the des- 

 cending and ascending columns, the ascending column being the warmer. 



In regions where volcanism, or mechanical action, or both, have 

 recently occurred, the difference in density resulting from difference in 

 temperature between the descending and ascending columns is likely to be 

 a much more important influence in the circulation of the ground waters 

 than in regions where the difference in temperature is due to the normal 

 heat of the rocks. Such a region is the Yellowstone Park. 



