b INTRODUCTION. 



besides wood preserved, in this way, such as shells, corals, or 

 sponges. 



The replacement of the original substance of a fossil by 

 some foreign body is thus a matter of common occurrence, 

 but it is by no means always easy to determine whether or 

 not such replacement has taken place. By far the com- 

 monest mode of replacement is that whereby an originally 

 calcareous skeleton is replaced by silica. This process of 

 " silicification " of the replacement of lime by silica is 

 not only an extremely common one, but it is also a readily 

 intelligible one ; since carbonate of lime is an easily and flint 

 a hardly soluble substance. It is thus easy to understand 

 that originally calcareous fossils, such as the shells of Mol- 

 lusca, or the skeletons of Corals, should have in many cases 

 suffered this change, their carbonate of lime being dissolved 

 away, particle by particle, and replaced by precipitated silica, 

 as they were subjected to percolation by heated or alkaline 

 waters holding silica in solution. 



When we meet with fossils, such as those alluded to 

 above, which we know to have been originally calcareous, 

 but which we now find, unchanged in form, but converted 

 into flint, then we cannot doubt that we have to deal with 

 cases of " silicification," and that the primitive skeleton of 

 lime has in these cases been slowly, and more or less per- 

 fectly, replaced by silica. We cannot, however, speak in 

 such a positive manner as to fossils which we now find to 

 be composed of flint, but as to the original constitution of 

 which we cannot be certain. We find, namely, some fossils 

 which are of uncertain affinities, and which are sometimes 

 found in a siliceous and sometimes in a calcareous state. 

 If we are not positive as to the zoological position of these 

 fossils, or if they belong to a group of animals in which we 

 find the living forms to possess sometimes a calcareous and 

 sometimes a siliceous skeleton, then it is obviously a matter 

 of extreme difficulty to determine whether the extinct forms 

 were really composed of lime or of flint. In such cases, we 

 must be guided principally by the condition of preservation 

 of the fossils which occur associated with such obscure forms 

 in the same beds ; the fact that the associated remains are 



