PART I. CHAPTER IV. 59 



Flint of Silicifiecl Fossils, whence derived. 



It is well known that the water of springs, or that which is 

 continually percolating the earth's crust, is rarely free from a 

 slight admixture either of iron, carbonate of lime, sulphur, flint, 

 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 silicified wood and 

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

 In other instances, as in tripoli and chalk-flint, it may have been 

 derived in great part, if not wholly, from the decomposition of 

 infusoria, sponges, and other bodies. But even if this be granted, 

 we have still to inquire whence a lake or the ocean can be con- 

 stantly replenished with the siliceous matter so abundantly with- 

 drawn from it by the secretions of these zoophytes. 



In regard to carbonate of lime there is no difficulty, because 

 not only are calcareous springs very numerous, but even rain- 

 water has the power of dissolving a minute portion of the calca- 

 reous rocks over which it flows. Hence marine corals and mbl- 

 lusca may be provided by rivers with the materials of their shells 

 and solid supports. But pure silex, even when reduced to the 

 finest powder and boiled, is insoluble in water. Nevertheless 

 Dr. Turner has well explained, in an essay on the chemistry of 

 geology,* how the decomposition of felspar may be a source of 

 silex in solution, as widely spread as are the felspathic rocks 

 which form so large a proportion of the volcanic, plutonic, and 

 metamorphic rocks, and are therefore universal, occurring some- 

 where in the course of every large river.'' 



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 flint. But this residue of the 

 decomposed mineral, which in its purest state is called porcelain- 

 clay, is found to contain only a small portion of the silica 

 which existed in the original felspar. The other part therefore 

 must have been dissolved and removed; and this can be 

 accounted for in two ways, first, because silex when combined 

 with an alkali is soluble in water ; secondly, because silex in 

 what is technically called its nascent state is also soluble in 

 water. Hence an endless supply of silica is afforded to the 

 waters of the sea. 



* Jam. Ed. New Phil. Journ. No. 30. p. 246. 



