3] The American Geologist. May. isos 
derived from eruptive rocks, and tliis origin has been substantiated 
by the United States Geological Survey, as may be seen in the 
text of the Sacramento and Placerville sheets, which are now being 
published, and which form part of the series of the geological maps 
of the Gold Belt now being issued under the authority of Mr. G. 
F. Becker. The author is probably right also in regarding some 
of the granite as later than the serpentine. That it is later than 
some of the serpentine appears certain from its cutting off the 
serpentine belt to the southeast of Placerville, as may be seen on 
the Placerville geological atlas sheet; and a dike of granitoid rock 
is intrusive in the serpentine area that lies two and a half miles 
northeast of Oleta in Amador county. Mr. Fairbanks states that 
some of the granite is later than the Mother lode slates, since south 
of Mariposa it has cut off and metamorphosed them, and the Mari- 
posa slates at Folsom have been found by Mr. Lindgren to have been 
altered by the intrusion of the granite. 
The fact that the large white masses forming portions of the 
Mother lode are not entirely quartz, but consist in part of a white 
magnesian mineral resembling dolomite, is referred to. This was 
first brought out by professor Whitney, who, to account for the 
occurrence of these large masses, writes as follows:* 
"But this immense mass of quartzose, dolomitic and magnesitic 
material, to which the name 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 3'et 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 largeh^ dolomitic in char- 
acter. " 
On page 217 Mr. Fairbanks presents a somewhat similar theor}' 
as follows: "That those portions of the lode so enormously ex- 
panded are simply coarse basic dikes of no great regularity or 
continuity, which, l^'ing in the course of the fissure, have been 
acted upon in a peculiar way by the penetrating liquids and gases. 
These, through metasomatic processes, have removed part of the 
original constituents and substituted others. A strong confirma- 
tion of this theory is found in a large body of unquestionably 
eruptive rock, near Jamestown, Tuolumne count}-, and about half 
*Auriferous Gravels of the Sierra Nevada, p. 332. 
