the Displacement of Shore-lines. 489 



If we should attempt to establish geological formations by 

 the aid of the known remains of terrestrial animals and plants, 

 the bomidaries of these would not coincide with those which 

 are defined by marine animals *. Thus, to mention an example, 

 the appearance of Dicotyledons does not coincide with the 

 boundary of any formation ; but they first appear with a 

 number of forms in the Upper Cretaceous period (Oeno- 

 manian). 



From Tertiary times, with the exception of the deposits in 

 the great mountain-chains, we know only formations of shallow 

 seas. The Tertiary deposits which correspond to the deep-sea 

 strata of older formations, and which were deposited further 

 from the land during that period without their formation being 

 interfered with by the numerous minor oscillations of the coast- 

 lines, still remain for the most part concealed from us in the sea. 

 Land-formations, freshwater and littoral formations such as 

 we have in abundance in our Tertiary basins, are greatly ex- 

 posed to destruction, for they are more frequently elevated above 

 the protecting sea. In the older cycles such formations are 

 more rare, probably to a great extent because they have been 

 destroyed by denudation. We may therefore conclude that 

 the Tertiary formations would much more resemble those of 

 the older cycles if our knowledge of them all were equal. Of 

 the older cycles we often know especially the deep-water for- 

 mations, of the youngest chiefly those of more shallow waters. 

 At some far distant period the exposed Tertiary formations 

 will come to equal those w^hich are now visible from older 

 cycles. 



Dawson (/. c. pp. 176-179) expresses the notion that the 

 remarkable regularity with which such cycles recur may 

 perhaps have a cosmical cause and be conditioned by one 

 or another astronomical period. But he seems afterwards to 

 reject this idea, because the Palaeozoic cycles have deposits 

 which are four or five times as thick as the Mesozoic (/. c. 

 p. 19.5), and we might therefore believe that more time must 

 have been occupied in their formation. But, on the other 

 hand, he notes that in Palseozoic times changes in the organic 

 world went on much more slowly in relation to the formation of 

 deposits than subsequently, so that the fossils extend through 

 greater thicknesses of strata than in the thinner, newer cycles. 

 If I were to judge from these facts adduced by Dawson, I should 



* •' The gi'owth of our knowledge concerning the terrestrial faunas and 

 floras of ancient geological periods has constantly forced upon the minds 

 of many geologists the necessity of a duplicate classification of geological 

 periods, based on the study of marine and terrestrial organisms respec- 

 tively." (J. W. Judd, loc. cit. p. 427.) 



