ORIGIN OF POTSDAM QUARTZITES. 93 



by heated waters, the unchanged condition of the intermediate and adjacent 

 sandstones cannot be accounted for, and the uniformity of the quartzites in 

 thickness and their marked separation from the associated common sand- 

 stone are equally difficult to understand. The quartzitic layers are some- 

 times separated by over one hundred feet of unchanged sandstone from the 

 underlying schists, while even the basal conglomerate has not been meta- 

 morphosed in the least particular. In sections of the Potsdam, also, where 

 the relations to the underlying schists are the same, and where there is no 

 evidence of greater elevatory or igneous action in the one case than in the 

 other, the sandstones of one exposure will contain numerous beds of 

 quartzite and those of another none. Evidently some other explanation is 

 required, and it appears to me that the true one must involve some peculi- 

 arity of the original deposition of the strata. 



It is well known that many sandstones contain a considerable percent- 

 age of silica in soluble condition, which on exposure becomes insoluble and 

 hardens them. Such sandstones when first taken from the quarry may be 

 cut with ease but become harder on lengthened exposure and are then 

 worked with much more difficulty. Again, scattered over the country 

 from Maine to Georgia and westward to Nevada, are found numerous 

 deposits of infusorial earth (composed largely of the silicious shields of 

 diatoms) which contain from 2 to 75 per cent of silica in a form readily 

 soluble in alkalies. The flint found so often in large beds in the Coal 

 Measures and in masses in other formations often contains fossil diatoms 

 composed of a soluble form of silica Besides this, the numerous silicious 

 deposits of springs on the Arkansas, the Yellowstone, etc., prove that silica 

 in a soluble state is not an unfrequent ingredient in rocks, nor unusual as a 

 present natural product. We know, too, that certain acids will throw down 

 dissolved silica in a gelatinous state, that certain alkalies will precipitate it 

 in the form of insoluble silicates, and that in the case of quarried sandstone 

 soluble silica is rendered insoluble by simple exposure to the atmosphere. 

 May not these quartzites, therefore, be the product of the cementation of 

 sands by the simple action of waters holding silica in solution, by the 

 admixture of the silicious portions of organisms, or in a manner similar 



