848 A TREATISE ON METAMORPHISM. 



The source of the silica for organisms is probably in part that dissolved 

 in sea water. Since, however, when filtered the pure sea water contains 

 only one part in 250,000, Murray and Renard "consider the probability of 

 the pelagic organisms which secrete silica obtaining it from the hydrated 

 silicate of alumina or clay held in suspension as well as the silica held in 

 solution. This might exjjlain the fact that these organisms abound in 

 brackish waters, and waters of low salinity and low temperature, where the 

 clay is more abundant than in the warmest and saltest waters of the ocean. 

 In the case of siliceous sponges, which are rooted for the most part in the 

 oozes and clays, Messrs. Murray and Renard think that the silica of their 

 skeletons may be derived from the silica in solution in sea water, or from 

 the colloid silica set free during the decomposition of the feldspathic rock 

 fragments and minerals in the deposits." 11 



Chemical deposits of silica may be formed when underground solutions, 

 especially hot solutions, reach the surface. The silica, in consequence of 

 the decrease in pressure and temperature, is precipitated. Such deposits 

 are known as siliceous sinter. The best known examples are the geyserite 

 formations of the Yellowstone Park. 



The silica for chemical deposits is believed to be mainly derived from 

 that contributed to the underground waters by the process of carbonation 

 in the zone of katamorphism, and especially in the belt of weathering, as 

 fully explained in Chapter VI (pp. 475-480). But it is certain that a por- 

 tion of the silica of the underground waters is directly derived by solution of 

 amorphous and semicrystalline silica of organic deposits and by the solution 

 of quartz itself. It is frequently supposed that crystallized quartz is not 

 dissolved by the ground waters, but, as shown by Hayes, 6 quartz crystals 

 at the surface are dissolved and corroded, and observations in many mines 

 show that the solutions have dissolved quartz in large quantities from the 

 wall rocks. This is illustrated by the granite of the Portland mine, at 

 Cripple Creek, Colo., where the quartz and mica are dissolved and the 

 feldspar is comparatively untouched. While, therefore, silica for chemical 

 precipitation is derived directly from the solution of quartz, a vastly 

 greater amount is the colloidal silicic acid liberated by the process of 



"Walcott, C. D., Fossil medusas: Mori. IT. S. Geol. Survey, vol. 30, 1898, p. 20. 

 & Hayes, C. W., Solution of silica under atmospheric conditions: Bull. Geol. Soc. America, vol. 8, 

 1897, pp. 213-220. 



