INFUSORIAL DEPOSITS. 223 
formation and deposition of the gravel beds. Much of it being ground up into 
fine powder, and becoming mixed with foreign material, assumed on deposi- 
tion and consolidation the form of clay, more or less homogeneous according 
to circumstances. In other cases the ashes seem to have fallen to considera- 
ble depth and to have become consolidated, without having undergone much 
movement from their original position, and to have been but little, if at all, 
acted on by water. 
The thickness of this deposit, like that of the other members of the vol- 
canic capping of the gravel series, is very irregular. At Webber Hill, near 
Placerville, the “ white lava” is from seventy-five to a hundred feet in thick- 
ness. On the divide between the South and North Webber creeks, near 
Burns’s Ranch, this material has perhaps a greater development than has 
been observed anywhere else in the mining region of the Sierra; it there 
attains a thickness of from 200 to 300 feet. 
It is in connection with the “ white lava” that the infusorial beds, — so far 
at least as the writer’s observations extend — are almost exclusively found. 
The portions which contain organic forms are interstratified with the other 
non-fossiliferous materials, and seem not to be always capable of being distin- 
guished from them by the unaided eye. Usually, however, the layers which 
are rich in microscopic forms are not only extremely fine-grained, but are 
also light and porous, resembling commercial magnesia in appearance ; indeed, 
the name magnesia is frequently given to this substance by the miners.* 
It is fine enough to make an excellent polishing powder, for which purpose 
more or less of it has been brought to San Francisco and sold. It does 
not seem, however, for some reason or other, to have ever come into general 
use, either in California or elsewhere.t 
The most interesting locality within the mining region where infusorial 
material has been quarried for economical purposes is one near Newtown, 
which was examined by Mr. Goodyear, and from whose notes the following 
description is taken. 
About a mile above the house of Mr. Samuel Fleming, which is two miles 
* To the writer’s knowledge the chemists and assayers of San Francisco have frequently called this 
infusorial silica “ magnesia,” when it has been submitted to them by miners and others for examination. 
Since it has become somewhat extensively used as a polishing powder it has received the name of “ electro- 
silicon.” 
+ The “rotten stone” of commerce, so much used for polishing, is a soft material resulting from the 
decomposition of impure silicious limestone. “Tripoli” and “Bath brick” seem generally to contain 
more or less infusorial silica. 
