52 MINERALOGY AND LITHOLOGY, 
38. OPAL. 
An amorphous and usually hydrous form of silica. 
This substance exists in large quantities in New Hampshire, in the 
condition of infusorial earth, or mountain meal, as it is often called. In 
trade it is called tripolite. The deposits of this substance are large, 
especially in the northern part of the state; and it is in that condition 
of purity that makes it the best polishing powder. The following is my 
analysis of a specimen from Lake Umbagog: 
Silica, . : F : : : : A a a ‘ ‘i 80.53 
Alumina, ‘i . z ‘ . . , c a . ‘ 5.89 
Iron sesquioxide, . : é é ‘ z - zi : F 1.03 
Lime, % : : - ‘i F ‘ e “ . . ‘ +35 
Water, . e : . , ‘ ‘ 5 - ‘ é ‘ II.05 
Organic matters, . : a: . . : . F 7 . -98 
99-83 
The analysis indicates that this substance is essentially hydrated 
silica. It is found forming layers in the muddy bottoms of ponds, and 
in bogs as a sub-peat deposit, and often it exists in such a state of purity 
as to be white and soft when dry, and thus it often attracts attention. 
These infusorial earths were first investigated by the Count Ehren- 
berg, and he found them to be nearly entirely made up of the siliceous 
remains of diatoms and infusorial animalcule, which, although of such 
minute size, possessed forms of great beauty. From a large bed of this 
earth near Richmond, Va., over one hundred species of these microscopic 
organisms have been obtained and described by Ehrenberg and Bailey. 
The New Hampshire deposits are, however, all fresh water deposits, and, 
in common with such deposits all over New England, they are entirely 
composed of diatoms, and contain no foraminiferal forms. Specimens 
from our state have been investigated by nearly every one who has inter- 
ested himself in the study of infusoria, but no better idea can anywhere be 
obtained of the appearance of these deposits under the microscope, and 
of their average composition, than by an examination of the figure which 
was drawn long ago by Ehrenberg, from a specimen of such an earth from 
our state. His work is not now easily accessible, and therefore I repro- 
duce the figure here. Fig. 6, on Pl. 4, represents the field of Ehrenberg’s 
