FOSSILISATION. 7 



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 commonest 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 origin- 

 ally calcareous fossils, such as the shells of Mollusca, or the skeletons 

 of Corals, should have in many cases suffered this change, long after 

 their burial in the rock, 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. 



In a large number of cases of silicification, the minute structure of the 

 fossil which has been subjected to this change is found to have been 

 more or less injuriously affected, and may be altogether destroyed, even 

 though the form of the fossil be perfectly preserved. This is the rule 

 where the silicification has been secondary, and has taken place at some 

 period long posterior to the original entombment of the fossil in the 

 enveloping rock ; whereas if the original fossilisation has been effected 

 by infiltration with silica in the first instance, then the minute structure 

 is usually perfectly preserved. In secondary silicification, as seen in 

 corals and shells, the carbonate of lime of the original fossil is gradually 

 more or less completely replaced by silica, the process beginning on the 

 exterior and gradually extending inwards. In the first stage of the pro- 

 cess, the outer layer of the fossil very commonly becomes more or less 

 largely converted into, or covered by, small circular deposits of silica, 

 having the form of a central boss surrounded by one or more concentric 

 rings (" orbicular silica " or " Beekite markings "). If the process goes 

 on, the whole of the fossil may ultimately become converted into flint. 

 Secondarily silicified fossils, though ill adapted for microscopic exam- 

 ination, are often of great beauty, as they commonly " weather out " from 

 the more readily soluble limestone in which they are embedded, and 

 can thus be obtained in absolute entirety. 



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, although 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 perfectly, replaced by silica. We cannot, however, speak in 

 such a positive manner as to fossils which we now find to be com- 

 posed 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 sometimes occur 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 



