146 THE FORMATION OF FLINTS 



Notwithstanding this evidence both chemists and 

 geologists were far from giving immediate acceptance 

 to the conclusions just set forth ; they found it difficult 

 to believe that a substance so insoluble as silica could 

 have been so completely displaced by a substance such 

 as carbonate of lime, which is more soluble than itself. 

 No doubt the difficulty was due largely to the fact that 

 the distinction between the opal of sponge skeletons 

 and the quartz of the mineral world w r as not sufficiently 

 kept in view ; but even with the full recognition of this 

 difference, a real difficulty still remains, for there can be 

 no question that under ordinary circumstances opal is far 

 less soluble in rain-water than carbonate of lime. 



Whatever may be the difficulties in the way of explana- 

 tion, there is no longer any doubt as to the facts. The 

 number and variety of skeletons originally siliceous but 

 now found to consist of carbonate of lime is too great 

 to admit of any other interpretation, and the solution of 

 opal as well as its replacement by carbonate of lime has 

 become one of the commonplaces of geological teaching. 



But if we turn to the chalk in the immediate vicinity 

 of a bed of flints this will yield us additional evidence of 

 the fact that sponge skeletons have passed into solution. 

 We have only to examine the freshly-fractured surface of 

 a piece of the chalk under a simple pocket lens, and we 

 shall perceive a number of hollow cavities having very 

 definite forms : some are circular, the transverse sections 

 of long cylindrical holes which extend into the rock; 

 others are elongated triangles, evidently longitudinal 

 sections of the same cavities. Sometimes at the broad 

 end of these triangles three shorter ones are seen pro- 

 ceeding from it, forming as it were a three-pronged fork. 

 Sometimes the longitudinal section tapers away at both 

 ends. As a result of these observations we conclude that 

 we have before us cavities of very definite shape ; some 



