220 C. W. HAYES — SOLUTION OF SILICA 



and keeping the rock surfaces moist, so that their solvent action might 

 be practically continuous for considerable periods. 



Conclusions. 



It seems probable, therefore, that the solution of the geodes and peb- 

 bles, concerning which there can be no question, was effected in the 

 manner outlined above. While it is granted that such agents seem in- 

 adequate to produce the effects observed, no others suggest themselves. 

 Moreover, as shown by Julien,* the activity of these agents is probably 

 not generally recognized, nor their capacity both for producing specific 

 and striking effects, such as those described above, and for effecting im- 

 portant geological changes upon a larger scale. If the above conclusion 

 be correct, a solvent capable of removing a third or half of a quartz peb- 

 ble an inch in diameter while still embedded in its matrix must be an 

 extremely important factor in gradation when acting under the much 

 more favorable conditions prevailing in the humus layer itself, where 

 the surface exposed by the quartz grains is vastly greater in proportion 

 to their bulk and where the solvent action is not interrupted as it must 

 be on exposed rock surfaces. 



*A. A. Julien: On the geological activity of the humus acids. Proc. Am. Assoc, vol. xxviii, 1879, 

 pp. 311-440. 



