344 GEOLOGY, 



part to barriers constructed by the sea itself, in the form of shoals, 

 bars and spits. In some cases the separation of the provinces may 

 have been due to nothing more substantial than sea-currents and the 

 attendant differences in temperature, or to locally increased or lessened 

 salinity of the waters. The formation and shifting of provinces due to 

 surface warpings seems to have been most marked in the Appalachian 

 tract, where deformations have been more pronounced than in the 

 interior in most geological stages. Some notable progress has recently 

 been made in working out these provinces, but the delineations have 

 not yet been brought to sufficient completeness and sureness to warrant 

 an attempt to sketch them here. 1 



(3) Cosmopolitan development. — Notwithstanding the local and 

 provincial modifications just named, the progress of the Ordovician 

 life on the American continent seems to have been, on the whole, in 

 the direction of cosmopolitanism. This was due, in part, to the wide 

 development of the epicontinental seas, which in themselves gave a 

 broad and rich field for evolution, and in part to the facilities for the 

 intermingling of the faunas of the various continents. A cosmopolitan 

 tendency is particularly marked in the great interior region which was 

 more stable than the border tracts and which better discloses the great 

 lines of biological movement, because less affected by the little ones. 



The preceding statements relate chiefly to the shallow-water life of 

 the sea, as this includes the larger part of all that is represented in 

 those portions of the deposits that are now accessible. The deep-sea 

 beds of the Ordovician are nearly all, probably quite all, inaccessible. 

 The shallow-water life that was subject to local and regional modification 

 embraced not only the species which were limited to particular kinds 

 of bottom, etc., but those which were dependent on these for food and 

 for other necessary conditions of life. 



Fortunately, however, in the Ordovician deposits there is an excep- 

 tional record of the free-floating life of the ocean made by an extinct 

 order, the graptolites (Fig. 167) ? This record is not, however, isolated, 

 as it might have been if deposited on a sub-oceanic plateau in mid- 

 ocean, inaccessible to coast life, and covered with water too shallow for 



1 See Paleozoic Seas and Barriers, by E. O. Ulrich and Charles Schuchert, Rept 

 N. Y. State Paleontologist, 1901, pp. 633-658. 



2 It is not universally agreed that all graptolites were floating forms at all stages, 

 but there seems to be little doubt that they usually were in their young stages at least. 



