RELATIONS OF DIFFUSION TO CEMENTATION. 637 



Superimposed upon these vertical movements are -lateral movements. 

 During the downward course under normal conditions the temperature and 

 the pressure steadily increase; during the upward course they steadily 

 decrease. But there are irregular variations in temperature and pressure 

 due to orogenic movements and to igneous intrusions. Increase of tem- 

 perature and pressure increases the capacity of water to hold material 

 in solution. The amount of material required to produce saturation is 

 therefore exceedingly variable for different portions of the underground 

 circulation. Equilibrium in the solutions extending between two places 

 depends not upon the absolute quantity of material contained, but upon 

 the relative approach to saturation at the various intermediate points. 

 Material migrates by diffusion from places where the approach to saturation 

 is nearest to places where it is less near, and this might be from a cool 

 dilute solution to a warmer stronger solution. The conditions of equilibrium 

 are therefore not those of uniform concentration, or in many cases even 

 approximately so, for often the temperature and the pressure are different 

 in horizontal columns from those in vertical columns, and vary greatly 

 in vertical columns if they are of great altitude. Low temperatures 

 and pressures obtain near the surface, and high temperatures and pressures 

 obtain at depth. Consequently as depth increases more material may 

 be held in solution before reaching a state of saturation; and because 

 of this the conditions of equilibrium are those of greater concentration 

 in the lower than in the upper parts of the columns. Moreover, the exper- 

 imental work of Barus," showing the wonderful dissolving power of water 

 on glass at high temperatures and pressures, but still at temperatures which 

 obtain in the lower part of the zone of fracture, shows that the differences 

 in the amount of material held in solution may be very great — may, indeed, 

 vary by ratios as great as 1 .: 100 or 1 : 1,000. 



From the foregoing it follows that if there were uniform concentra- 

 tion, with variable temperatures and pressures in the underground solutions, 

 the material would diffuse from high levels to low levels and thus produce 

 equilibrium. Where the currents are moving downward, but with decreas- 

 ing slowness on account of the lateral movement of the water, which 

 continually carries a part off to one side (see pp. 572-576), the lower 



« Barus, Carl, The compressibility of hot water and its solvent action on glass: Am. Jour. Sci., 3d 

 ser., vol. 41, 1891, pp. 110-116. 



