WATER AS A CHEMICAL AGENT. 709 



They have also produced extensive deposits of silica in regions of 

 not springs, remarkable examples of which occur in the Yellowstone 

 Park, and also in Iceland and New .Zealand. The silica in these de- 

 posits is mostly in the state of common opal. When the depositions 

 cease, from the failure of the hot waters, much of the material soon 

 crumbles, and loses its peculiar external features. 



Wood, shells, and insects, are often petrified by such siliceous 

 waters, so that silicified stumps are common over large portions 

 of the Pacific slope, one of the most remarkable regions of igneous 

 eruption in the world. Portions of trunks of more than a hundred 

 silicified trees, one of them twelve feet in diameter, lie prostrate to- 

 gether, according to Marsh, in a thick bed of tufa, about five miles 

 southwest of Calistoga Hot Springs, in the Coast Range, north of San 

 Francisco, California — a locality first made known by C. H. Denison. 

 The trees are described as probably all Conifers. They received the 

 silica from the tufaceous deposit, and probably while it was penetrated 

 by heat and moisture, if not comprised within the range of true hot 

 springs ; and the tufa had its origin in a shower of volcanic cinders . 

 (from some unascertained vent) settling down over the forest region. 



Siliceous solutions have moreover silicified the fossils of many of the 

 earth's limestones and other strata, and made flint or hornstone nodules 

 in them, though without silicifying the limestone itself. 



The most of the above results have been produced by hot, or at 

 least warm, solutions. But, in the case of the fossils and hornstone in 

 limestones, — of which the chalk affords an example, — even a low 

 heat could hardly have been necessary. The silica was distributed 

 through the calcareous mud of the sea bottom, in the form of Diatoms, 

 Polycystines, and siliceous spicules of Sponges, and therefore was in 

 the soluble state ; and the solution of this silica took place within the 

 mass of the deposit. The tendency of matter of one kind to concrete 

 together led to the forming of flint-nodules and the silicifying of shells 

 and other foreign substances. 



3. Through Oxy (Lotion. — The oxydation of the iron of ferriferous 

 minerals, in the destruction of rocks described above, is also a forma- 

 tive process. It usually results, as has been stated, in making the 

 brown hydrous oxyd, limonite, unless either the climate is a dry one, 

 or the temperature is near or above the boiling point, when the red 

 oxyd, hematite, is formed. Further, accumulations of iron ores in 

 great beds have been thus made. Carbonates containing iron and 

 sulphids of iron have been the chief sources of the ore ; but, where 

 these were present to start the process, all other iron-bearing minerals 

 at hand have contributed to the end. 



In a large number of cases, the rock has decomposed and left the 



