THE SILICIFICATION AM) INDURATION OF ROCKS. "221 



spaces to some small degree are filled with other materials ; and because it 

 cannot be asserted that they are ever perfectly filled, although apparently 

 this is often the case. This very large theoretical amount of silica is ap- 

 proximated in the somewhat rare, evenly granular, pure, vitreous quartzites. 

 It is certain that the amount of secondary quartz required to indurate such 

 vast formations as the Paleozoic and pre-Paleozoic quartzites of the west is 

 enormous. 



The thicknesses of the Weber, Ogden, and Cambrian quartzites of the 

 Wasatch, using Emmons' and King's lowest estimates, aggregate 18,000 

 feet.* The Uinta sandstone and quartzites have an estimated thickness of 

 from 10,000 to 13,000 feet.f The quartzite of the Medicine Bow mountains 

 of Wyoming is of great, although undetermined, thickness.^ The combined 

 area covered by these quartzites is thousands of square miles. An exami- 

 nation of my collection of specimens and thin sections from all of these 

 regions shows that the chief cause of the induration of the rocks is interstitial 

 quartz, the major part of which has been added to the original clastic grains. 

 The quartz deposited in vein filling is as nothing compared with this. 



As to the source of these vast quantities of silica, we can at present do 

 little more than conjecture. It seems to be taken for granted by most writers 

 that quartz itself is wholly insoluble within the crust of the earth ; that in 

 order to be dissolved the silica must be in the colloid form. These are points 

 upon which evidence is needed. That much silica is derived from and taken 

 in solution during the decomposition of silicates cannot be doubted. We 

 know that silica is often largely contained in the water of hot springs.§ Is 

 it not probable that the water deep within the crust, therefore presumably at 

 a relatively high temperature, carries ordinarily a considerable quantity of 

 silica which is ready to be deposited when favorable conditions arise? Nu- 

 merous experiments upon crystallization show that the presence of crystallized 

 nuclei in a solution is very favorable for the deposition upon them of like 

 material. In the quartzites we have such nuclei in the rounded grains of 

 sand. 



In the elder Hitchcock's remarkably able studies upon the metamorphism 

 of rocks,|| published iu 1861, are described some extensive conglomerates 

 associated with and passing into crystalline schists, which are very similar 

 to those of the Black Hills. He had not the microscope to assist him ; yet 



*United States Geological Explorations of the Fortieth Parallel, Vol. I, Systematic Geology, by 

 Clarence King, 1878, pp. 155-156. 



t Ibid., p. 150; Geology of the Uinta mountains, .T. W. Powell, 1876, pp. 143-144. 



X United States Geological Explorations of the Fortieth Parallel, Vol. II, Descriptive Geology, by 

 Arnold Hague and S. F. Emmons, 1S77, pp. 104-109. 



2 For foreign localities, see Roth's Allegemeine und Chemische Geologie, Vol. I, 1879. For 

 United .states localities, see Bulletins of the U. S. Geological Survey., No. 32, Lists and Analyses of 

 the Mineral Springs of the United States, Albert C. Peale, and No. 47, Analyses of Waters of the 

 Yellowstone National Park, with an Account of the Methods of Analysis employed, Frank Austin 

 Gooch and James Edward Whitfield. The latter bulletin gives over forty water analyses, in all of 

 which silica is found. In many it constitutes twenty-five or more per cent, of the total soluble 

 material, while in one case it runs as high as fifty per cent. 



|| Geology of Vermont, Edward Hitchcock, Vol. I, pp. 22-52. 



