The inaugural Address. 



285 



foraminiferous life were developed and perished in building* their own 

 vast cenotaph of calcareous ooze. Soon, however, would a fresh era of 

 poriferous life begin ; a new protoplasmic scum would appear, and 

 again the great silicious sponges would rear their forms and float on 

 the calcareous ooze ; and so in oft repeated succession foraminiferous 

 and poriferous life would alternately wax and wane and leave the 

 records of their history in what is now the chalk cliff and its inter- 

 calated bands of flint. No doubt in the long struggle for existence 

 the w^axing and the waning of its own form of life would be gradual 

 in each case. As the sponge layer became denser the rhizopod layer 

 would find the conditions for its existence less favourable. But as 

 the former became more rigid the foraminifera would again assert 

 themselves in their former vigorous growth, and under the wreck of 

 their succeeding generations would eventually cover over and deeply 

 bury the silicified and still silicifying sponge-bed with chalky ooze. 



Such an explanation has much to recommend it to our minds as 

 a vera causa. Silica would be absent, or nearly so, from the chalk- 

 ooze, since the dialysing process would be continuous — nay, might 

 be continuing now in our chalk hills if there were still silicious 

 matter to be eliminated by rain water from the chalk. Indeed, a 

 process of this kind is going on in the action of the water on the 

 flint itself. Agate and flint alike, when exposed to the action of 

 weathering, that is to say, to the action of rain or river water, 

 charged as the former always is with carbonic acid and ammonia, 

 and as the latter generally also is with alkaline carbonates and salts, 

 undergo a change. Soluble silica — chalcedonic silica — is extracted 

 from them, and the surface of the agate or flint is eroded and be- 

 comes porous, the solid residue being silica that has become insoluble 

 in the manner I alluded to before as a probable result of a change 

 very slowly affecting the original chalcedony. The sponge-like 

 surface, however, is now found to have its hollows filled with car- 

 bonate of lime and other ingredients foreign to the original mineral. 



There is still much to be done in completing the explanation of 

 the origin of flint. The sketch I have given represents, I think, 

 the most probable account of the matter from our present point of 

 view. The history of a curious silicious substance that occurs in 



VOL. XXI. — NO. LXIII. U 



