Geology of the Mother Lode gold belt. — Fairbanks. 213 
The diabase extends through Amador county in several par- 
allel dikes, the western one of which is nearly two miles wide. In 
El Dorado county it gives place for a number of miles to a beauti- 
ful syenite porphyry, which in turn is replaced north of Placerville 
by irregular discontinuous dikes of diorite and diabase to the 
middle fork of the American river, where these dikes become very 
numerous and in conjunction with serpentine finally take the place 
of the black slates. As the Mother Lode approaches this point it 
becomes less distinct. There being no confining bodies of rock, on 
the east the veins spread out in the schists where they are usually 
barren. At Oregon Bar the Mother Lode may be considered as 
really terminating. Here all its chai'acters are lost and the gold 
instead of occurring in quartz is found in clay seams in the de- 
composed crystalline rocks. 
Although at times the diabase seems to blend into the adjoining 
uncrystalline schists and frequently shows lamination yet it can be 
considered nothing else than an eruptive rock, for in many places 
it has an amygdaloidal structure. The amygdules occur both in 
the massive portions and in the matrix of the conglomerates. The 
fragmental portions, which occur over nearly the whole extent of 
the outburst, have been classed as tuffs or metamorphic conglom- 
erates by former geologists, but this is incorrect if we accept 
Kosenbusch's definition of a tuff. 
Serpentine is one of the rocks almost alwa}'s associated with the 
Mother Lode, sometimes forming one of the walls for man}' miles. 
Its influence does not tend to produce a well defined vein, the 
quartz bodies being almost always bunchy and irregularly placed. 
When the serpentine occurs in bodies of great thickness it is gen- 
erally massive, but the long, narrow, dike-like forms are lamin- 
ated. From the appearance of the lamination it is evident that it 
has been produced by pressure and movement and is not due to 
original sedimentation. G. F. Becker, in his report on the quick- 
silver deposits of the Pacific slope, has advanced the theoiy that 
all the serpentine of the coast range has been derived from sedi- 
mentary rocks, chiefly sandstones, through metasomatic processes, 
and that those of the gold belt have originated in the same manner. 
As far as my observation in the Mother Lode region has gone I see 
not the slightest reason for attributing to it any such origin. 
The occurrence of the serpentine in long, narrow bodies inter- 
secting the slates and crystalline rocks, and in bunches in the 
