42 FLINT OF SILICIFIED FOSSILS. [Ch. IT. 



percolating the earth's crust, is rarely free from a slight admixture either 

 of iron, carbonate of lime, sulphur, siHca, potash, or some other earthy, 

 alkaline, or metallic ingredient. Hot springs in particular are copiously 

 charged with one or more of these elements ; and it is only in their 

 waters that silex is found in abundance. In certain cases, therefore, 

 especially in volcanic regions, we may imagine the flint of siHcified 

 wood and corals to have been supplied by the waters of thermal springs. 

 In other instances, as in tripoli, it may have been derived in great part, if 

 not wholly, from the decomposition of diatomacese, sponges, and other 

 bodies. But even if this be granted, we have still to inquire whence a lake 

 or the ocean can be constantly replenished with the calcareous and siliceous 

 matter so abundantly withdrawn from it by the secretions of living beings. 



In regard to carbonate of lime there is no difficulty, because not 

 only are calcareous springs very numerous, but even rain-water, when 

 it falls on ground where vegetable matter is decomposing, may be- 

 come so charged with carbonic acid as to acquire a power of dis- 

 solving a minute portion of the calcareous rocks over which it flows. 

 Hence marine corals and mollusca may be provided by rivers with 

 the materials of their shells and solid supports. But pure silex, even 

 when reduced to the finest pov/der and boiled, is insoluble in water, 

 except at very high temperatures. Nevertheless Dr. Turner has well ex- 

 plained, in an essay on the chemistry of geology,'*^" how the decomposi- 

 tion of felspar may be a source of silex in solution. He has remarked 

 that the siliceous earth, which constitutes more than half the bulk of 

 felspar, is intimately combined with alumine, potash, and some other 

 elements. The alkaline matter of the felspar has a chemical affinity for 

 water, as also for the carbonic acid which is more or less contained in 

 the waters of most springs. The water therefore carries away alkaline 

 matter, and leaves behind a clay consisting of alumine and silica. But 

 this residue of the decomposed mineral, which in its purest state is called 

 porcelain clay, is found to contain a part only of the silica which existed 

 in the original felspar. The other part, therefore, must have been dis- 

 solved and removed ; and this can be accounted for in two ways ; first, 

 because silica when combined with an alkali is soluble in water ; sec- 

 ondly, because silica in what is technically called its nascent state is also 

 soluble in water. Hence an endless supply of silica is afforded to rivers 

 and the waters of the sea. For the felspathic rocks are universally dis- 

 tributed, constituting, as they do, so large a proportion of the volcanic, 

 plutonic, and metamorphic formations. Even where they chance to be 

 absent in mass, they rarely fail to occur in the superficial gravel or allu- 

 vial deposits of the basin of every large river. 



The disintegration of mica also, another mineral which enters largely in- 

 to the composition of granite and various sandstones, may yield silica which 

 may be dissolved in water, for nearly half of this mineral consists of silica, 

 combined with alumine, potash, and about a tenth part of iron. The ox- 

 idation of this iron in the air is the principal cause of the waste of mica. 



* Jam. Ed. New Phil. Journ. No. 80, p. 246. 



