METHODS OF CORRELATION 1143 



Mississippic and in the Middle and early Upper Jurassic (Callovian 

 to Oxfordian), as well as in the early part of the Cretacic. 



(2) The stages of initial diastrophism and sea-retreat are 

 marked by the increase in deposition of the material resulting from 

 the deep decomposition of the rock, during the period of base- 

 leveling; the increasing deposition of terrigenous elastics, and the 

 consequent change in the character of the fauna, a turbid water 

 fauna taking the place of the one previously flourishing in the 

 purer waters. The littoral belt is narrowed, the epicontinental seas 

 disappear, and the evolution of shallow-water life and the migration 

 of organisms are restricted. 



(3) When the climax of the regression is reached the restric- 

 tion in the evolution of the shallow water organisms is at its maxi- 

 mum. Clastic deposits predominate and they even encroach upon 

 the continents. Climatic changes are toward a colder period, ow- 

 ing to the locking up of the carbon dioxide in land- vegetation, by 

 solution of limestones, and by carbonation of silicates. Such refrig- 

 eration may go so far as to result in glaciation, at least locally, the 

 evidences of such glaciation furnishing an additional basis for cor- 

 relation. Broad land expansions would in general favor wide dis- 

 tribution of animals and plants, unless the severity of the climate 

 should enter in as a deterrent factor. Should climatic factors be less 

 in evidence, however, the wide expansion of the land might favor 

 a wide distribution of land life. As a result, the struggle for ex- 

 istence would be less intense and modifications would be slower, 

 and more of the nature of adaptations to slowly changing environ- 

 ment. The littoral region of the sea being, however, much reduced, 

 the survivors of the once widespread marine littoral fauna would 

 be forced into a more restricted area and hence a fierce struggle 

 for existence would be sure to result. This would lead to the rapid 

 extinction of numerous types and to the comparatively rapid modi- 

 fication of the survivors, and would thus produce a comparatively 

 sudden change in the character of the fauna. 



(4) The early stages of quiescence and base- leveling which fol- 

 low and which initiate anew the slow transgressive movement 

 of the sea, will again favor migration of marine faunas. Owing to 

 the effect of the previous restrictions, however, the aspect of the 

 fauna will have been changed to a marked degree, so that the ex- 

 panding fauna will have a distinct aspect of its own. The great 

 expansion which followed the retreat of the sea in the late Middle 

 or early Upper Cambric time in North America brought with it 

 the spread of a fauna widely dififerent from that which preceded it. 

 The Middle and early Upper Ordovicic expansional faunas (Chazy- 



