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, 80.53 



Alumina, 5.89 



Iron sesquioxide, 1.03 



Lime, .35 



Water, 11.05 



Organic matters, .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 animalculae, 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 



