FOSSILS. 9 



in one sense metamorphic, as all have undergone some 

 changes, however trifling they may be, since their formation ; 

 but they are not properly termed metamorphic unless their 

 original structure is much obscured. At the same time, it is 

 a remarkable fact that in many regions we find an alternation 

 of rocks highly metamorphosed with others that have suffered 

 comparatively little change.^ 



The stratified rocks generally contain remains of the plants 

 and animals which existed at the time of their formation. Of 

 the latter the most conspicuous are the hard parts of Mollusca, 

 Crustacea, stony Corals, and Sea-urchins, and the bones and 

 teeth of Fishes, Reptiles and Mammals. These are termed 

 fossils. The rocks are also liable to contain the organic remains 

 of pre-existing periods, just in the same way as fossils are 

 often found mingled with the recent shells in our present 

 shore-deposits. These derived fossils are often spoken of as 

 Remanics. They may frequently be detected by their rolled 

 or fragmentary condition, by being accompanied by a portion 

 of the matrix of the rock in which they were first embedded, 

 or by their occurring in pebbles. The fossils found in the 

 Triassic pebble-bed of Budleigh Salterton furnish a remark- 

 able instance of derived fossils ; and it has been questioned 

 whether the parent rocks of some of them are represented in 

 England. The shells found in the " Box stones " at the base 

 of the Crag in Suffolk belong to an earlier Pliocene period, 

 which is not known for certain to be represented in our 

 country. These are examples of " the Fragmentary strata of 

 England." 



The fossils proper to a formation furnish us with a key to 

 its age, and the mode of its accumulation, whether by fresh- 

 water, estuarine, or marine agencies. In identifying a deposit 

 by its organic remains, it must, however, be borne in mind 

 that not only do the sedimentary conditions of one period 

 vary in different places, but that the forms of life whose exist- 

 ence depends largely upon the conditions, likewise vary in 

 different localities. " Distinct faunas may be separated by 

 narrow barriers in existing seas ; and differences almost as 

 great may occur on the same coast-line without the inter- 

 position of any barrier, merely in passing from a sea-bed of 

 rock and weed to one of sand or mud, or to a zone of different 

 depth. It would be unreasonable to expect the same fossils 

 in a limestone as in a sandstone ; and even in comparing 

 similar strata we must consider the probability of their 



1 J. J. Bigsby, Edin. New Phil. Journ. (2), April, 1863 ; and J. C. Ward, 

 Q. J. xxxii. 30. See also 'The Study of Rocks,' by Frank Rutley, 1S79. 



