GEOLOGY. 
43 
less purely siliceous varieties, are usually full of small brown scales, 
spines, and fragments or impressions of nondescript shape which are 
of organic origin but which can not be recognized as belonging to any 
particular forms. Here and there, also, large bones are embedded 
in the deposits. They seem to be those of whales or other large marine 
vertebrates. 
Taken as a whole, the Monterey shale may well be called an organic 
formation. The practically complete absence of coarse sediments 
derived from erosion and the abundance of fossil organisms, espe¬ 
cially of siliceous skeletons, make it different both in appearance and 
composition from any other known formation of comparable thick¬ 
ness. The unaltered siliceous shale most nearly resembles chalk, but 
it contains only a small proportion of lime. Whether or not the 
organic remains compose more than half or as much as half of the 
deposit can not be stated. 
MICROSCOPIC APPEARANCE. 
Under the polarizing microscope little can be made out regarding 
the structure of the main mass of the soft shale and compact white 
shale. The groundmass seems to be made up of amorphous colloidal 
silica surrounding minute grains which are both crystalline and amor¬ 
phous, but the character of which can not be recognized. Embedded 
in this are numerous imperfectly angular or more rarely partially 
rounded crystal particles, probably of quartz. Many of the latter 
look as if they were due to secondary development rather than origi¬ 
nating as clastic grains. 
In the more flinty varieties the rock appears to have undergone par¬ 
tial and local crystallization of the silica throughout its mass. In the 
flint, in which there are alternating, usually crumpled bands of opaque 
light-colored flint and clear amber-colored or black flint, the opaque 
bands are composed mainly of amorphous material like that of the 
softer shale, but in a much more compact state, and the translucent 
bands are mainly crystalline aggregates. The opaque bands include 
crystalline particles and, locally, patches of crystal grains like those 
of the clear flint, and they are included longitudinally by intermittent 
bands of the clear flint. Furthermore, they are in many specimens 
of a patchy appearance, parts being less amorphous than others. 
The bands of the clear flint are composed chiefly of small grains of 
crystalline quartz, and these are surrounded by a finer grained aggre¬ 
gate of crystalline and amorphous particles. The quartz grains have 
neither the rounded outlines of waterworn grains nor angular crystal¬ 
line outlines, but are branching, and appear more like growths. Angu¬ 
lar patches of the amorphous silica, many of them showing signs of 
