26 THE AURIFEROUS GRAVELS OF THE SIERRA NEVADA. 
Fifth. The Cretaceous rocks occupy but small areas in the southern Coast 
Ranges. As we proceed north, we find them coming in, in force, in the 
Monte Diablo Range, gradually widening northwards, soon forming the bulk 
of that range, and to the north of the Bay of San Francisco becoming almost 
the exclusive formation. It is not abundantly supplied with fossils, except 
at certain localities of limited area. By far the larger portion of the Coast 
Range Cretaceous is entirely destitute of organic remains. 
Sixth. While the larger portion of the Tertiary of the Coast Ranges is but 
little metamorphosed, there has been a great deal of chemical change in the 
Cretaceous since its deposition. The products of this metamorphism are 
chiefly imperfect jaspers, serpentine, and mica-slates. The latter grow more 
and more abundant as we go toward the northeastern extremity of the 
system, beyond Clear Lake. The large amount of metamorphism by the 
introduction of magnesian combinations is especially remarkable. These 
metamorphic bands and patches do not appear to follow any system in their 
distribution ; at least, none such has hitherto been discovered. A large 
amount of study will have to be bestowed on these rocks before their nature 
will be well understood. 
Seventh. The principal upheaval and disturbance of the Coast Range sys- 
tem must have taken place at the close of the Miocene epoch. The Miocene 
and the Cretaceous seem everywhere to be conformable with each other, 
although there is a large area of the last-named formation over which it 
does not appear that any more strata were ever deposited, and which we 
must suppose to have been elevated above the sea-level before the Tertiary 
began to be formed. The break between the Miocene and Pliocene is very 
marked in portions of the ranges; in other parts there is a complete con- 
formability between these two formations. This is particularly the case on 
the inside —the eastern side —of the Coast Ranges, as is well seen in all 
the sections across the Monte Diablo Range to the San Joaquin plains. 
It follows therefore that the disturbances which took place after the depo- 
sition of the Pliocene were somewhat local in character, and that they were 
chiefly felt on the side of the mountains nearest the Great Valley. The 
break at the Golden Gate, the predominance of volcanic rocks from there 
north, for a considerable distance, and the powerful disturbances of the Plio- 
cene in the region just south of the Bay of San Francisco may be connected 
together as being probably the results of one’ general cause. 
