INTRODUCTION. 



granular and not a crystalline form, and it is therefore very 

 difficult to account for the state of preservation of these speci- 

 mens, unless we admit that the skeleton was primitively silice- 

 ous, and that we have here a case of the substitution of the 

 hardly soluble silica by the easily soluble carbonate of lime. 



In any case we must carefully distinguish between replace- 

 ment, 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, calcareous, or argil- 

 laceous mud at the bottom of the sea, the surrounding 

 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 car- 

 bonate 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. All the natural 

 cavities of a fossil, even down to the minutest microscopic 

 pores or tubes, may in this way become filled with some 

 such infiltrated material, the two commonest agents in this 

 process being lime and flint. If the skeleton of the fossil 

 be a calcareous one, while the infiltrating material has been 

 some less soluble substance, such as silica or some silicate, 

 then the skeleton may be artificially or naturally dissolved 

 away, leaving a cast of the internal cavities of the fossil 

 formed of the infiltrated matter. Thus the minute shells of 

 Foraminifera are often infiltrated with the silicate glau- 

 conite, and exquisitely perfect casts of their interior cavities 

 are subsequently formed by dissolution of the shell itself. 

 In this way, as we shall see hereafter, deposits of green 

 sand have been sometimes produced. 

 



DEFINITION OF EOCK. 



The crust of the earth consists of various different ma- 

 terials, produced at different successive periods, occupying 



