364 ON THE MOVEMENTS OF THE EARTh's CRUST. 



Oligoceue is again distinguished by a negative phase. In the latter 

 part of the Oligoceue period, and still more during the Miocene, the 

 sea again rose ; between the Miocene and Pliocene it retreated far, and 

 at the beginning of the Quaternary epoch it rose again. Similar great 

 oscillations are also to be traced in North America and in Patagonia. 

 But marine Miocene deposits are wanting in the last-mentioned locality, 

 where the Miocene fresh-water beds are associated with great quantities 

 of volcanic products. 



At the commencement of the Tertiary period, when the sea had re- 

 treated far under high latitudes, the climate of Europe was temperate 

 rather than tropical (see Sa[)orta, Le Monde des Plantes avant Vappari- 

 tion de Phomme, 1879). According as the sea rose and the Eocene over- 

 flow advanced the climate became warmer, and at the close of the 

 Eocene period the climate of Southern Europe was hot and dry. The 

 abundant Tertiary flora of the Arctic lands is (according to Saporta 

 and Gardner) rather Eocene than Miocene (as Heer supposed). At the 

 boundary between Eocene and Oligocene the sea retreated, and the 

 Arctic Tertiary flora began to migrate into Europe, sui)planting the 

 more southern plants. Then came the Miocene overflow, and with it a 

 rich tropical or sub-tropical flora. But in proportion as the Miocene sea 

 retreated, the European flora also, little by little, lost in richness and 

 beauty and the tropical elements became more and more rare. During 

 the Pliocene epoch the sea retreated still farther, and the climate be- 

 came colder and colder until the Glacial period came in. But the last 

 Quaternary overflow has again, after several oscillations, caused the 

 ice to retreat, and our climate has again become temperate. There is 

 thus clearly" a relation of dependency between the climate and geo- 

 graphical conditions. Great seas under high latitudes produce warm 

 climates, and vice versa. 



Now we have seen that these great geographical changes were in all 

 I)robability a consequence of the risingandsinkingof the mean value of 

 the eccentricity, and we must therefore believe that these great changes 

 of the climate had a cosmical origin, and occurred at the same time 

 over the whole earth. We still know too little of the geology of tropical 

 countries ; but there is ground for the belief tliat here also great changes 

 have taken place in the distribution of land and sea, and that these 

 changes must also have had an influence upon the climate of the warm 

 countries. 



It is further probable that the force of vulcanicity stands in relation 

 to the changes in the eccentricity. Each of the great geological forma- 

 tions, from the Pre-Cambrian itself, has had its volcanoes (see A. Geikie, 

 " Textbook," pp. 259-2G0) ; and we have already seen that the same 

 author states that there have been periods in the earth's history when 

 vulcanicity was much more powerful and widely distributed than at 

 other times. We have seen how the upheaval of the land was accom- 

 panied by volcanic outbursts ; and as regards the Tertiary period, at 



