OSIDES 103 



The Alabama and Georgia deposits, like those of Beaux, are 

 regarded as of chemical origin.^ 



The material from yarious sources varies greatly in chemical 

 composition, as shown by the analyses on page 102. 



Silica. — Silica, as has been already noted under the head of 

 rock-forming minerals, is one of the most abundant constituents 

 of the earth's crust. In its various forms, which are sufficiently 

 extensive to constitute rock masses, it is always of chemical 

 origin, that is, results by deposition from solution, by precipi- 

 tation, or evaporation, as noted above. Varietal names are 

 given to the deposits, dependent upon their structure, method 

 of formation, color, and degree of purity. Siliceous sinter, 

 geyserite, and -fioriie are names given to the nearly white, 

 often soft and friable, hydrated varieties formed on the evapo- 

 ration of the siliceous waters of hot springs and geysers, or 

 through the eliminating action of algous vegetation. The ma- 

 terial is, in reality, an impure form of opal. Throughout the 

 geyser regions of the Yellowstone Park, Iceland, and New 

 Zealand, the sinter has been deposited as a comparatively thin 

 crust over the surface, or in the form of cones about the throats 

 of the geysers. The varieties of silica known as opal are hydrous 

 forms occurring in veins and pockets, in a variety of rocks. 

 Frequently it forms the replacing material in silicified or '* petri- 

 fied'' woods. In the old lake beds of the Madison valley, Mon- 

 tana, are found large logs composed wholly of this material, no 

 sign of organic matter remaining, but yet with the woody struc- 

 ture beautifully preserved. Chalcedony is the translucent, mas- 

 sive, cryptocrystalline variety of silica occurring mainly in 

 cavities in older rocks, where it has been deposited by infiltra- 

 tion. It is a common secondary product formed during the 

 decomposition of many rocks, and, like opal, may form the 

 petrifying medium of fossil woods and other organisms. Not 

 infrequently, also, it occurs in continuous layers of several inches 

 or even feet in thickness, interstratified with limestone. Flint 

 is a variety of chalcedony formed by segregation in chalky lime- 

 stone, and is composed, in part, of the broken and partially dis- 

 solved spicules of sponges, and the siliceous casts of infusoria. 

 Chert is an impure flint. It occurs in rounded, nodular, con- 

 cretionary masses interbedded with limestones, particularly 



^See rimmi of the subject, by E. L. Packard, in Mineral Besources of 

 tlie United States for 1891. 



