FOSSILISATION. 7 



converted into flint pointing to the probability that the prob- 

 lematical forms were originally calcareous, and vice versa. 

 In the" case, also, of all fossils which present themselves 

 sometimes in a siliceous and sometimes in a calcareous form, 

 there is always the presumption that the skeleton was origi- 

 nally composed of lime, this presumption being based upon 

 the fact that the conversion of the calcareous skeletons of 

 animals into silica by a process of replacement is an un- 

 questionable, an extremely common, and a readily intelligible 

 occurrence. 



Until recently, indeed, naturalists never allowed them- 

 selves to contemplate the alternative possibility of an origi- 

 nally siliceous skeleton being replaced by lime ; but we have 

 now high authorities in favour of the view that this anom- 

 alous mode of substitution really does occur occasionally. 

 The only case in which anything like definite proof of this 

 abnormal mode of replacement can be said to have been 

 brought forward is that of some of the fossil sponges found 

 in the chalk. Certain of these sponges agree in their 

 general form and in their microscopic structure with the 

 living Hexactinellid Sponges (such as the Venus's Flower- 

 basket), the skeleton of which is composed of flint, and it is 

 therefore difficult to doubt the possession by the fossil types 

 of a primitively siliceous skeleton. We might, indeed, sup- 

 pose that the fossil sponges in question represented an en- 

 tirely new type of sponge-structure with the form of the 

 living Hexactinellids, but with a calcareous skeleton, and 

 that this skeleton had in the process of fossilisation been often 

 replaced by flint ; but this supposition would be a violent 

 one, and would not be warranted by the known facts. It 

 has, however, been pointed out by Zittel that these sponges 

 are sometimes found in the rocks with their skeleton in part 

 calcareous and in part siliceous ; and the same distinguished 

 observer has further shown that in such specimens the 

 siliceous portions of the skeleton retain their minute micro- 

 scopic character, whereas the calcareous portions have en- 

 tirely lost their minute structure, and are composed simply 

 of crystalline calcite. Had the skeleton been originally 

 calcareous, the lime composing it would have been in a 



