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 
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.t 
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 sulphurets 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 ratonale 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 ante, pp. 46, 49. 
t The Great Quartz Vein presents one of the most interesting fields for study offered to the chemical geologist 
in any country. 
