Geology of the Mother Lode gold belt. — Fairbanks. 221 
It may be seen from the foregoing statements that the crystal- 
line rocks must be younger than the slates, and hence it is de- 
cidedly eiToneous to classify them as Archaean. The granite 
cannot be considered metamorphic, for whatever was its original 
condition its present one is that of a truly eruptive rock. 
The formation of the Mother Lode is the final event in the 
history of these rocks. No dikes intersect it and the fissure has 
broken through all the formations that lie in its path. 
The alterations which the rocks have undergone are remarkable. 
The pressure created during the. mountain making movements has 
been the chief factor in producing this result. The rocks are 
nearl} r all laminated, the exceptions being the granite and portions 
of the serpentine and diabase. 
The coincidence of the original bedding of the clastic rocks, 
with the schistose structure produced by pressure is a source of 
great difficulty in distinguishing their origin. This is quite re- 
markable in the case of the black slates in which it is shown, by 
the position of the fossils, that the cleavage has not been superin- 
duced by pressure but is that of the original sedimentation. It is 
likely that a far greater portion than is usually supposed of the so- 
called metamorphic rocks are really eruptive. 
No detailed microscopic examination of the Mother Lode rocks 
has yet been made, however some general facts have been eluci- 
dated. The feldspar of the granite and some of the dikes is com- 
paratively fresh, but with these exceptions the decomposition is so 
great that it is impossible to determine the species of plagioclase 
feldspar or distinguish it from orthoclase. Distinctl}' orthoclase 
feldspar appears only in small amounts even in the granite. While 
the rocks as a whole are equally remarkable for the almost entire 
absence of quartz. In but few instances was there noticed any 
tendency toward the separation of distinct feldspar crystals in the 
diorites or diabases. In the diabase the augite is almost always 
present in idiomorphic crystals. The} r exhibit a beautiful zonal 
■structure and in their decomposition are bordered by fibrous green 
hornblende. The greenish matrix is decomposed and indefinite 
in character. The hornblende of the diorites does not usually 
appear in well-formed cn'stals. 
The serpentine alwa} r s shows its derivation from a former crys- 
talline rock. It is not certain what the character of that rook was. 
though from traces of a bronzy lustre in the former pyroxene 
