1887.] 



Note on Professor HuWs Paper. 



309 



hardened." Also that "the formation of flint may therefore be 

 regarded as due to the separation of silica from sea- water, and its 

 deposition round some organic body in a state of chemical change or 

 decay." 



This is essentially the theory I advanced in our joint paper, and 

 that independently arrived at by the Abbe Renard, namely, pseudo- 

 morphism. 



Dr. Nicholson says further : " It has been asserted that the flints of 

 the chalk are merely fossil sponges. No explanation of the origin of 

 flint, however, can be satisfactory, unless it embraces the origin of 

 chert in almost all limestones from the Silurian upwards, as well as 

 the common phenomenon of the silicification of organic bodies (such 

 as * corals and shells) which are known with certainty to have been 

 originally calcareous" 



In our paper the prevalence and thickness of the chert of the 

 Carboniferous Limestone of Ireland is referred to. I have since had 

 an opportunity of seeing the siliceous alteration of limestone on a 

 very large scale, and in different formations, in the tropical region of 

 Western Australia, when engaged there as Government Geologist. It 

 is seen in the Lower Silurian, Carboniferous, and Upper Tertiary 

 deposits. The transition from the limestones into chert, flint, and 

 calcedony, is clearly visible in many places where these minerals 

 form ranges often miles in extent, and where the thickness of the 

 flinty material occasionally reaches 300 feet. 



It is curious that these flint beds nearly always form the capping 

 of the hills, but that they are of the same formation as the under- 

 lying limestone is proved by the gradual passage of that rock into 

 flint; and where fossils occur in the limestone similar fossils are 

 observed in the flint, until they become obliterated towards the 

 summit.* I am inclined to attribute this to the action of highly- 

 heated rain-water since the rocks have been deposited. In the warm 

 season — which is also the rainy season, from about November to 

 March — the rocks become intensely heated, and consequently, also the 

 water lying in pools and cavities. I have been assured by settlers 

 who have had to wade through flooded country, that at such times they 

 could hardly endure the heat of the water, and I have experienced this 

 to a slight extent myself. It is certain that under these circumstances 

 silica would be more largely dissolved fi'om one part, and more 

 quickly deposited in another portion of the same rock ; it is in fact 

 on similar reasoning — the influence of sunlight and heat — that 

 Professor Martin Duncan, P. U.S., explains the silicification of the 

 West Indian Miocene Corals. 



* See 'Keporton the Geology of the Kimberley District, W. Australia.' E. T. 

 Hardman. Perth, W. A., 1885. P. 18. 



