WATER AS A CHEMICAL AGENT. 693 



strata. Leaves, stems, and nuts are often petrified by the calcareous 

 waters. 



The waters dripping into limestone caverns produce, by their calca- 

 reous depositions, the pendent stalactites of the roof of the cavern, and 

 the stalagmite of the floor. The stalagmite shows, in a cross frac- 

 ture, the fact of its gradual deposition, by the bandings in its colors. 

 The deposit from such waters sometimes has a soft chalky texture. 



Some sand and clay beds owe their consolidation to carbonate of lime 

 derived from the remains of shells present in them. 



2. Through Siliceous Waters. — Siliceous waters have done far the 

 larger part of the consolidation of sandstones, conglomerates and clay 

 beds. The silica has commonly been taken up from feldspars distrib- 

 uted throughout the rock itself, or from the siliceous relics of Diatoms 

 and Sponges present in it, by the heat and moisture penetrating it ; 

 and then consolidation has taken place as the temperature lowered. 

 Such solutions have filled fissures and cavities in rocks with quartz, 

 making quartz seams and veins. They have also been the means by 

 which mineral silicates have been formed in the process of meta- 

 morphism (p. 726). 



They have also produced extensive deposits of silica in regions of 

 hot 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 



