220 THE AURIFEROUS GRAVELS OF THE SIERRA NEVADA. 
respect. Their abundance and importance made it desirable that they should 
be specially investigated, and such specimens as the writer could procure 
were submitted to Mr. Lesquereux, the eminent authority in fossil botany, 
for examination and description. His work constitutes Part IL. of this vol- 
ume, and will of course be referred to by those who are interested in the 
various questions involved in the discussion of the gravels of the Sierra. 
The animal remains found in the auriferous detrital series are also of great 
interest and importance. They naturally divide themselves into two classes, 
for the purposes of such a work as the present, which does not profess to be 
paleontological in character. Each of these classes will be considered in a 
separate section, one of which will be devoted to human remains and works 
of human hands, the other to remains of animal origin and not human. 
The reasons for the different treatment which these various subdivisions of 
this chapter here receive at the writer’s hand will be apparent after a peru- 
sal of the following pages. 
Section Il. — The Infusorial Rocks of the Auriferous Gravel Series. 
Probably the most suitable, and certainly the most convenient, term to use 
for speaking of the strata containing microscopic organisms is to call them 
by the general term “infusorial,” which does not demand or imply an ad- 
herence to any particular theory of their origin and biological affinities. 
Under the term “infusorial rocks,” therefore, we include all those deposits 
which are made up in part of bodies or fragments of bodies of organic origin, 
but so small as only to be visible under the microscope. As the bodies in 
question are chiefly of a silicious nature, it is very common to speak of 
the aggregation of such as “ infusorial silica.” 
It is chiefly owing to the labors of Ehrenberg that we have become ac- 
quainted with the geological importance of the infusorial element in rocks 
of various kinds and ages in extensive and widely separated regions. The 
various publications of this eminent naturalist on this subject will be found 
in part in the Proceedings and Memoirs of the Berlin Academy of Sciences. 
The magnificent folio entitled “ Die Mikrogeologie,” published in 1854, con- 
tains a complete résumé of all the results attained by Ehrenberg in the ex- 
amination of rocks, soils, dust, ashes, and other masses or accumulations of 
matter from every part of the globe. Among the vast variety of materials 
here described were some collected by Fremont in Oregon, in the region of 
