10 INTRODUCTION 



great value in determining, as of the same age, deposits 

 found in widely-separated localities. 



In addition to their chronological value, fossils are 

 also important in indicating the conditions under which 

 the formations were deposited. In the case of the later 

 beds, where most of the fossils belong to genera which are 

 still existing, it is easy to distinguish a marine deposit 

 from one formed in freshwater or on land. Even in the 

 rocks of earlier periods, in which most of the genera are 

 extinct, we may recognise a marine deposit by the presence 

 of such animals as radiolarians, corals, echinoderms, 

 brachiopods, pteropods, cephalopods, or cirripeds, which at 

 the present day are found only in the sea. 



The depth of the sea in which a formation was deposited 

 can be estimated when the fossils belong to living species ; 

 when the species are extinct some idea may be formed if 

 the genera to which they belong are found chiefly at some 

 particular depth at the present day. In attempting such 

 determinations it must be remembered that the sea-bottom 

 down to a depth of nearly 50 fathoms may be disturbed by 

 the action of waves and currents in the sea ; consequently 

 the animals living on the bottom in shallow water are 

 liable to be carried from their original home to higher or 

 lower levels. One of the surest indications that a formation 

 was laid down in shallow water and not far from land is 

 furnished by the association of the fossil remains of land 

 animals and plants with marine species; another, by 

 the presence of mollusks such as Pholas, Saxicava and 

 Lithodomus, which bore into rocks, and at the present day 

 are found only in shallow water. 



The nature of the climates of past ages may be judged 

 to some extent by the character of the fossils ; the evidence 

 furnished by land-plants is particularly valuable, since 



