60 MINERALOGY, 



at ordinary temperatures to some extent in all parts of 

 the globe, as is shown by its deposition in grasses and 

 other plants, and in the textures of animals also, especial- 

 ly some of those minute animals called infusoria, as you 

 will see in another part of this book. These deposits 

 are of course made from solution, the solvent being the 

 sap in vegetables and the blood in animals. 



117. Silica in two States. There are two states, then, 

 in which silica exists, the soluble and the insoluble. Most 

 of it in the world is in the insoluble state, in the rocks 

 and pebbles, and sand and grains in the soil ; but some 

 of it is changing continually from this state into the solu- 

 ble, and as continually passing back again by deposition. 

 Thus, by the agency which I have mentioned, some of 

 the silica in the soil (which came originally from the 

 rocks) is dissolved in the water, and finds its way into 

 the plant by the sap, where it is deposited in the texture, 

 and when deposited it is insoluble again. If the plant 

 decay or is burned the silica is returned again to the 

 earth. The same succession of changes we have also in 

 relation to some animals. And it is supposed that the 

 insoluble silica which we have in chalcedony, jasper, etc., 

 is a deposit from the soluble state. 



118. Petrifactions with Silica. When wood decays in 

 water which contains considerable silica in solution, pet- 

 rifaction occurs ; that is, as the particles of wood are re- 

 moved in the process of decay, particles of silica are de- 

 posited in their place. Of course the silica is deposited 

 from a solution of it that penetrates the wood. The ar- 

 rangement of the wood is preserved, so that it appears 

 as if the wood had been turned into stone. Some speci- 

 mens are exceedingly beautiful when sawn across what 

 was the grain of the wood and polished. 



119. Silicates of Lime. These are of so little import- 

 ance that I need not describe them. The same is true 

 of the boro-silicates that is, compounds of silicic and bo- 

 racic acids with lime combined together in one mineral. 



