482 GEOLOGY. 



do within the first few hundred feet of their courses below the water- 

 level, they are liable to pass through a succession of dissolving and 

 depositing stages, each reaction resulting in a state that makes a new 

 reaction possible. This is especially true if the waters pursue deep 

 courses. Strictly speaking, the precipitations usually concern only a 

 part of the substances dissolved. New substances are often taken up 

 in the very act of throwing down those already held, and the way thus 

 prepared for further changes. If the water pursues a deep and devious 

 course, it may receive additions by solution and suffer losses by pre- 

 cipitation at many points in its course, both descending and ascending. 

 The changes are very complex, and in the case of a deep or long circuit 

 where various rocks, pressures, and temperatures are encountered, 

 the history becomes one long succession of complexities, the full nature 

 of which is not yet revealed. 



In the deeper circuits, each individual current usually takes on a 

 descending, a lateral, and ascending phase, the three being necessary to 

 complete a circuit. The chemical conditions of the waters in the three 

 phases are probably not sharply distinguished from one another, and 

 hence there seems to be no defined horizon of concentration comparable 

 to that near the water-level already described. The chief distinctions 

 in the deeper regions relate to pressure, temperature, length or depth 

 of penetration, and duration of contact. It seems safe to assume, as a 

 general truth, that, other things being equal, the solutions become more 

 complex and more nearly reach general saturation the farther and the 

 deeper the waters penetrate. 



It has long been a mooted question whether ore-deposits are due 

 chiefly to descending, to lateral, or to ascending currents. The question 

 in its usual form is too undiscriminating for advantageous discussion, 

 but if the ore-deposits due to surface or short-course concentrations 

 and reconcentrations be set aside, as in some sense a separate class, 

 the relative functions of the descending, the lateral, and the ascending 

 portions of the deeper circulations become a measurably definite ques- 

 tion. Two great working factors enter into the comparison: (1) much 

 greater circulation in the upper zone, where lateral movement most pre- 

 vails; (2) much greater heat and pressure in the lower zone, where 

 the circulation must be chiefly vertical. 



Heat and pressure in general favor solution, and hence so far as this 

 factor goes, descending water is hkely to be increasing its mineral con- 



