A CENTURY OF GEOLOGY. 555 



but at the same time more or less complete obliteration of geo- 

 graphical diversity. 



4. After these periods of migrations and minglings there would 

 be re-isolations in new localities, and the process of diversification 

 would recommence and increase as long as the isolation continues. 



The last of these critical periods of migi'ations and minglings 

 and struggles for life among competing species was the Glacial 

 epoch or ice age. Therefore the present geographical distribution 

 of species was largely determined by the extensive migrations of 

 that time. 



n. Cosmopolitan and Provincial Faunas. — There are appar- 

 ently in the history of the earth periods of widespread or cosmo- 

 politan faunas, alternating with localized or provincial faunas. 

 The cosmopolitan periods are usually times of prevalence of lime- 

 stones or organic sediments, and the fossils are very abundant. 

 The provincial periods are usually characterized by sandstones and 

 shales or mechanical sediments, and are comparatively poor in fos- 

 sils. Moreover, it is believed that the cosmopolitan limestone 

 periods are oceanic periods — i. e., periods of wide oceans and lower 

 and smaller continents and little erosive activity, while the sand- 

 stone periods, characterized by provincial faunas, are periods of 

 higher and larger continents, and therefore of great erosion and 

 abundant mechanical sedimentation. 



!N"ow, according to Chamberlin, these remarkable alternations 

 are due to oscillations of the crust, in which the continents are alter- 

 nately lifted and depressed. It must be remembered that abyssal 

 faunas are almost unkno^vn among fossils. This is the necessary 

 result of substantial permanency of oceanic basins. The whole 

 geological record is in shallow-water faunas. These shallow waters 

 are along continental shore lines and in interior continental seas. 

 According to Chamberlin again, during a period of continental 

 depression all the flat continental margins are submerged, forming 

 broad submarine platforms, and the lower interior portions of the 

 continents are also submerged, forming wide and shallow interior 

 seas. Under these conditions continental waste, and therefore sand 

 and clay sediments, are reduced to a minimum. Life, animal and 

 vegetal, abounds, and therefore much limestone is formed. The 

 oceans are widely connected with one another, and therefore the 

 faunas are widespread or cosmopolitan. During the period of ele- 

 vation, on the contrary, the continents are extended to the margin 

 of the deep oceanic basins, the broad, shallow submarine platforms 

 are abolished, the interior seas are also abolished, the shallow- 

 water areas are reduced to isolated bays, and their faunas are 

 peculiar or provincial. Also, elevated and enlarged continents 



