FENNER, THE WATCHVNG BASALT 107 



above the preexistent lakes being favorable for percolation, the waters 

 would naturally tend to rise to the surface. A circumstance favorable to 

 this result lay in the fact that the lava retained a store of heat long after 

 consolidation. Immediately after the latter event the temperature was 

 so high that apparently all water which gained access was vaporized, not- 

 withstanding the impediments to escape caused by restrictions of the 

 channel; but as the temperature fell, the water level in this arm of the 

 system of circulation slowly rose. 



The system of circulation thus conceived is analogous to that of arte- 

 sian flows, only modified in this respect, that the efPective head was due 

 not so much to a difference in elevation between the points of ingress and 

 exit, as to vaporization of the water in the upper portions of the channels 

 of exit. It embodies the principle by which C. E. Van Hise has sought to 

 explain the deposition of metallic minerals in fissure veins.^^ Exception 

 has been taken to Van Rise's conception of the origin of metalliferous 

 deposits, and in this case, where the operation of the principle seems 

 almost demonstrable, the effects are distinctly different from what he has 

 conceived that they should be. In justice, however, attention should be 

 called to the fact that present exposures show only the lower portions of 

 the return channels, and the possibility that higher levels would present 

 more similarity to metalliferous veins is not necessarily excluded. 



An alternative h}^othesis to the one which has been offered should be 

 considered, that is, the descent of meteoric waters direct from the surface 

 through the channels in which alteration was effected. The form of this 

 h\-pothesis which would consider cold surface waters as the agent may be 

 dismissed almost at once ; the results are entirely at variance with it. At 

 tlie present time, such waters are effecting the solution of certain of the 

 minerals, are decomposing others to pulverulent or clayey masses, are 

 oxidizing the iron compounds to hydrated, rusty-looking material, and in 

 short are giving to the whole the appearance which is characteristic of 

 surface weathering. There still remains, however, the idea that the action 

 may have taken place while the rocks were still highly heated. Two chief 

 objections appear. The leaching which the rocks experienced while alter- 

 ation was in progress, as shown by the great reduction in iron and mag- 

 nesia contents, and the marked migration of other compounds implies a 

 fairly rapid circulation. It is difficult to conceive of the means by which 

 this could be secured under the hypothesis which we are now considering. 

 Secondly, the nature of the secondary minerals shows conclusively that 

 the process was not one of oxidation. Surface waters passing through 



» Trans. Am. Inst. Mln. Eng., vol. 30, pp. 27-177, 1900. 



