348 ON THE MOVEMENTS OF THE EARTh's CRUST. 



From Tertiary times, with the esceptiou of tlie deposits in the great 

 moiiiitaiu-cliains, we kuow only formations of shallow seas. The Ter- 

 tiary deposits which correspond to the deep-sea strata of older forma- 

 tions, and which were deposited farther from the land dnring that 

 period without their formation being interfered with by the numerous 

 minor oscillations of the coast-lines, still remain for the most part con- 

 cealed from us in the sea. 



Land-formations, fresh- water and littoral formations such as we have 

 in abundance in our Tertiary basins, are greatly exposed 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 formations, of the young- 

 est chiefly those of more shallow waters. At some far-distant period 

 the exposed Tertiary formations will come to equal those which are now 

 visible from older cycles. 



Dawson (/. c. pj). 17G-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 wiiich are four or five times as thick as the Mesozoic {I.e. 

 p. 195), and we might therefore believe that more time must have been 

 occupied in their formation. But, on the other hand, he notes that in 

 Pala?ozoic 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, 

 1 should come to a different conclusion ; I should regard it as a proba- 

 ble supposition that the formation of deposits went on more rapidly in 

 Pala'-ozoic times than later on. If the moon at that time were nearer to 

 us and the sidereal day shorter, as Darwin thinks, the tidal wave must 

 both have been stronger and have acted more frequently than at pres- 

 ent. The coasts would be destroyed much more rapidly, and the sea 

 would have much more material to deposit. A cycle of this period 

 would be thicker than the younger cycles, and the fossils would extend 

 tbrough a greater thickness of strata than in the latter. For I see at 

 present no probable ground for the supposition that the development of 

 new sjjccies would be accelerated in the same degree as the formation 

 of deposits. 



There is therefore reason to assume that it is owing to these great 

 changes in the form of the earth, occurring at long intervals, that we 

 can distinguish between geological formations. But such great changes 

 in the distribution of land and sea must necessarilv also bring with them 



