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-live to a hundred feet in thick- 



ness. 



Wet 



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 for 



at least as the writer's observations extend 



rn 



# 



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. f 



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. 



>» 



f 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. 



