8 INTRODUCTION. 



which we find the living forms to possess sometimes a calcareous 

 and at others 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 converted into flint pointing to 

 the probability that the problematical forms were originally calcare- 

 ous, 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 originally 

 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 unquestionable, an extremely common, 

 and a readily intelligible occurrence. 



Until recently, indeed, naturalists never allowed themselves to 

 contemplate the alternative possibility of an originally siliceous 

 skeleton being replaced by lime; but we have now unequivocal 

 evidence that this anomalous mode of replacement is of not very 

 uncommon occurrence. The researches of Zittel, Hinde, and 

 Sollas have, in fact, proved that the colloid silica of the siliceous 

 skeletons of the Flinty Sponges is comparatively unstable, and that 

 under certain circumstances it can be readily dissolved in water. 

 Hence these Sponges are commonly found in the fossil condition 

 with the silica of the original skeleton more or less extensively 

 replaced by carbonate of lime, or by oxide or sulphide of iron. 

 When the replacing agent is lime, it is found not only that the 

 microscopic structure of the original skeleton has been completely 

 lost ; but that the lime is always in the crystalline condition, consist- 

 ing of unoriented crystals of calcite. This latter fact affords conclu- 

 sive proof that the skeleton was not primitively calcareous, but that 

 the lime is of secondary origin and has replaced some other material 



In any case we must carefully distinguish between replacement, 

 whether by flint or any other mineral, and infiltration, the latter 

 being merely the process whereby the cavities and natural vacuities 

 of a fossil are liable to become filled by some mineral substance, 

 subsequent to its entombment in sediment. When such a fossil as 

 a shell or a coral, for example, becomes buried in the sandy, cal- 

 careous, or argillaceous mud at the bottom of the sea, the surround- 

 ing sediment often does not penetrate into the deeper parts of the 

 fossil, and there are thus left in its interior certain empty spaces, 

 into which the surrounding water makes its way by percolation. 

 Any mineral substances, such as carbonate of lime or silica, which 

 may be contained in solution in the water, are then liable to undergo 

 precipitation, and to be deposited in a solid form within the fossil. 



