



■V-: 















332 



RESUME AND THEORETICAL DISCUSSION. 



indicating powerful longitudinal compression of the whole body of the strata 

 in which it is enclosed.* But this immense mass of quartzose, dolomitic, and 

 magnesitic material, to which the name of Mother Lode, or Great Quartz 

 Vein, is applied, is not by any means proved to be a fissure vein, or even an 

 exclusively segregated one. It will require much more study than it has 



i 



yet received before its real character can be stated with confidence. To 

 the writer it seems, from present evidence, most likely that it is the result 

 of metamorphic action on a belt of rock of peculiar composition, and perhaps 

 originally largely dolomitic in character. That this belt should have under- 

 gone the same modifications of position suffered by the limestone formation 

 of the Sierra at the time of the upheaval of the chain, would be, then, quite 

 in accordance with what might be expected.! 



We admit, then, as being on the whole the most reasonable theory, 

 that the chemical changes in the gravel are the result, first, of volcanic 

 impregnation from beneath, and of subsequent alteration by the ordinary 

 meteorological agencies penetrating from above, and working downward as 

 opportunity offered. It now remains to indicate a little more exactly the 

 nature of these changes, which have resulted in the formation of the blue 

 gravel, and in the development from this of the red variety. The latter 

 stage of the process presents no difficulties, and may be rapidly passed over. 

 Wherever air and moisture together can readily find their way down to some 

 distance beneath the surface, there the rock becomes more or less completely 

 disintegrated, this result being brought about, in considerable part at least, 

 by the higher oxidation and hydratation of the oxides and sulphurcts pres- 

 ent in the mass. This is the phenomenon so often exhibited by metal- 

 liferous veins, which are, as a general rule, easily permeated by water, and 

 which are in consequence decomposed down as far as the line of permanent 

 water level. So far moisture and air easily penetrate ; below that the air 

 is almost excluded, and the ores remain in their original condition. The 

 resulting oxidized material is the well-known "iron hat" of the miner. 



It is a fact, the rationale of which, however, is not thoroughly understood, 

 that reducing agencies predominate in all those chemical operations which 

 are carried on at considerable depth, or which are propagated from below 

 upward ; while oxidation appears to be the invariable result of the access of 



* See mite, pp. 46, 49. 



t The Great Quartz Vein presents one of the most interesting fields for study offered to the chemieal 



in any country. 



geologist 







