18 STRATIGRAPHICAL GEOLOGY 



into fossils depends partly on the solidity and character of the 

 substance of which the hard parts are composed, and partly on the 

 rapidity with which the remains are covered up and embedded in 

 sediment. Lastly, their final preservation as fossils depends upon 

 the conditions to which the embedding rock is subsequently ex- 

 posed, upon its conservation or destruction. 



The remains of those creatures which live in deep water or over 

 deep water, such as many Foraminifera, Radiolaria, siliceous sponges, 

 and pteropods, have such delicate tests and spicules that they often 

 fall to pieces and become mere debris before they can be embedded 

 in sediment ; for in deep water the accumulation of sediment is 

 usually a very slow process. The thin shells of the Radiolaria 

 and the Pteropoda are especially liable to rapid disintegration, and 

 consequently they are rare as fossils, although they are known to 

 have existed from very early times. 



2. Subsequent Removal of Organic Remains. The proportion of 

 those remains which would otherwise be preserved is further 

 diminished by the subsequent destruction of some of them 

 by the action of percolating water. All water which percolates 

 downward from terrestrial surfaces contains carbonic acid, and such 

 carbonated water attacks and dissolves carbonate of lime, and also 

 that form of silica which enters into organic structures. Hence 

 all such embedded remains are liable to solution and removal 

 under certain circumstances. 



So long as the deposit containing embedded organisms remains 

 below the sea in which it was formed, it is probable that the 

 changes which take place are slight and gradual. It is when the 

 fossiliferous deposit is raised above the level of the sea and is 

 added to the dry land that changes are apt to take place. Even 

 then, if the rock has formed part of a thick series deposited during 

 a long period of subsidence, and has remained buried beneath 

 others for some time before the whole mass is raised to form 

 land, most of the fossils in it are likely to be preserved, for the 

 rock becomes consolidated and remains saturated with saline water, 

 which probably contains as much carbonate of lime as it is capable 

 of holding in solution under such conditions. Many fossils exhibit 

 changes of structure which have very probably been effected during 

 upheaval and while the rock was in a water-soaked condition ; 

 thus the substance of shells is frequently converted into crystalline 

 calcite, and in other cases the calcareous shell is replaced by 

 chalcedonic silica. 



Those parts of a formation, however, which come to form the 

 surface portions of newly raised land, i.e. those parts which are 

 raised above the lowest summer level of saturation, are subjected 



