CHEMICAL REACTIONS IN THE GRAVELS. 



329 





intense volcanic activity during the latter portion of the gravel epoch. 

 Most of the chemical reactions indicated above can therefore be accounted 

 for without difficulty by referring to the well-known results of that complex 

 series of conditions embraced under the term " volcanism." 



There are some points, however, in regard to which there is more diffi- 

 culty, and these will now be taken up for a brief discussion. The first of 

 these would be the question whether the chemical agencies connected with 

 the volcanic displays were effected from above downward, or in the reverse 

 direction. We find frequently large bodies of gravel overlain by equally 

 heavy masses of volcanic materials ; we almost never have the latter under- 

 lying the former. This, of course, is the natural result of the fact that the 

 culmination of the volcanic era took place toward the close of the gravel 

 epoch. Such being the case, the question arises whether the effect of an 

 overlying mass of lava would make itself perceptible downward to the 

 bottom of a deposit of gravel, perhaps several hundred feet thick. This 

 seems, on general principles, hardly probable, and an examination of the 

 conditions of the various beds which make up the whole mass of the forma- 

 tion, detrital and volcanic both being included, appears to confirm the idea 

 that the chemical changes which have taken place in the body of the gravel 

 cannot have been caused by any reaction originating in the overlying erup- 

 tive materials. Wherever the gravel is very deep and most confined within 

 a limited space, instead of being spread out over a wide extent of surface, 

 there it seems to have at the bottom most fully the msemMe of characters 

 designated by the term " blue." Had the chemical influences producing this 

 condition come from above, it would seem that, at least in some places, these 

 deeper portions would have failed to be reached. The same thing seems to 

 be indicated by the condition of the volcanic masses themselves. It is, as a 

 general rule the lower eruptive beds which exhibit the most marked indica- 





fe 



tions of chemical alteration. The upper layers are usually but very little 

 changed from their original condition, as far, at least, as can be made out 

 without microscopical examination. One of the most striking facts connected 

 with the mineralogy of the more solid upper portions of the volcanic forma- 

 tions of the Sierra is, the almost entire absence of the zeolitic and other 

 minerals, which are so often found in connection w T ith eruptive rocks in other 

 parts of the world. Hardly a specimen of any zeolite has ever been detected 

 in California, notwithstanding the large area there covered by formations in 

 which such occurrences might reasonably be expected. 



