16 A FLAKE OF FLINT AND ITS HISTOKT. 



chalk from a totally different source, or was introduced 

 subsequently to the deposition of the whole. 



I believe I am right in saying that there is still a current 

 notion among many persons who are not scientific geologists 

 that flint is a kind of igneous rock ; and its hardness and 

 superficial resemblance to some kinds of obsidian may at 

 first sight lend some countenance to such an idea. Never- 

 theless, it is almost superfluous to add, such a notion is a 

 totally erroneous one, and the mode in which flint occurs 

 without altering the limestone with which it is in contact, 

 or the fossils which it contains is of itself sufficient to 

 refute any such origin. 



From the hardness and insolubility of flint, it would 

 appear a very natural inference that silica in all forms is 

 likewise insoluble, but, as proved by siliceous springs, 

 silica in a certain condition is freely soluble in alkaline 

 waters. This naturally suggested to the earlier geologists 

 that flints had been deposited by the aid of hot springs or 

 heated waters in the cretaceous sea at certain intervals 

 during the formation of the chalk sea. Thus, the late 

 Dr Mantell wrote that " the nodules and veins of flints 

 that are so abundant in the upper chalk have probably 

 been produced by the agency of heated waters holding 

 silex in solution, and depositing it when poured into the 

 chalk sea." It will, however, be obvious that there are 

 very serious difficulties in accepting any such explanation. 

 In the first place, we should require the introduction of 

 floods of heated water containing silica at certain irregular 

 intervals .during the whole period of the deposition of the 

 upper chalk ; and it would also be essential that these waters 

 should have extended over vast areas of ocean. Secondly, 

 the mode in which nodular flint occurs could not 

 adequately be explained by any theory of this kind ; while 

 it would leave the origin of the tabular flint, which we have 

 seen may occur in joints and fissures, totally unaccounted 

 for. Another idea was that the silica had been, at least 

 partially, introduced from above after the deposition of the 

 chalk ; but, although this explanation may, and perhaps 

 does, partly account for the formation of some of the 

 upper tabular flints, it is quite impossible that it could 



