48 . GEOLOGY. 



Defined as it now is, the Triassic was a period of great transitions, in 

 which many types were inaugurated, but a lew only were carried to 

 their characteristic developments. 



The Marine Life. 



The physical description has made it clear that the withdrawal 

 of, the sea which restricted the marine life of the Permian was continued 

 into the Trias, during which it reached its climax, at least in North 

 America. There was then a very general emergence of what is 

 now land, and probably also of some tracts now submerged. The 

 marine life of the shallow- water type was therefore not only greatly 

 reduced, but because it occupied border tracts now buried, such record 

 as it made is mainly concealed from present examination; in other 

 words, there was not only less life, but we know less relatively about 

 what there was. This was not equally true of other continents, although 

 measurably true of all, so far as present knowledge extends. To follow 

 the continuity of the shallow-water marine life, it is necessary to bring 

 together evidence from different continents. The question of supreme 

 interest is the mode by which the epicontinental sea life, crowded to a 

 minimum between the land and the deep sea, maintained its con- 

 tinuity, transformed its species, and, emerging at length, re-peopled 

 the shallow waters when they again spread out upon the continental 

 platform in the closing stages of the Trias and in the Jura. 



When the sea readvanced on the North American continent, it 

 was primarily from the Pacific border, but was attended by incursions 

 along the Mackenzie Valley and from the Gulf of Mexico. It was 

 not till long after that an advance from the Atlantic made an accessi- 

 ble record. It is not clear that the sea ever completely withdrew 

 from the present land area on the Pacific border, but the fossils so 

 far recovered do not give clear evidence that there was there at all 

 times a harbor of refuge, or an embayment of shoal water of suffi- 

 cient area to develop, during the retreat of the sea, a definite provincial 

 fauna which subsequently spread with the advancing seas and made 

 itself felt as a pronounced faunal unit. Rather does the evidence 

 seem to point to a coastal tract merely, in which a restricted fauna 

 lived on and developed new species which migrated subsequently 

 as individuals, rather than as a faunal assemblage. 



The transition tracts. — It was otherwise on the eastern continent. 



