1 142 PRINCIPLES OF STRATIGRAPHY 



going. The shrinking of the land from invading lower temperatures 

 will likewise tend to reduce the level of the land, and so permit 

 the sea to transgress across it anew. 



If erosion is not uniform in amount, owing to variable hardness 

 of formations, or to other causes, the resulting hiatus will vary 

 in magnitude from point to point. For, though the time interval 

 between the retreat and readvance of the sea in two localities might 

 be the same, it is obvious that the missing formations will be more 

 extensive than can be accounted for by non-deposition, since, dur- 

 ing the interval of exposure, erosion removes a part of the earlier 

 deposited sediments. The amount thus removed in different sections 

 may vary greatly, and, hence, the gap in the series will vary from 

 place to place. 



Chamberlin (9:50./) has considered four stages, which must be 

 taken cognizance of in correlation by general diastrophic move- 

 ments : ( I ) "the stages of climacteric base-leveling and sea trans- 

 gression ; (2) the stages of retreat, which are the first stages of 

 diastrophic movement after the quiescent period; (3) the stages 

 of climacteric diastrophism and of greatest sea-retreat; and, (4) 

 the stages of early quiescence, progressive degradation, and sea- 

 advance." 



(i) The stage where base-leveling and sea transgression have 

 successively reached their climax is especially characterized by 

 diminution in land, a reduction in the amount of solution, oxidation 

 and carbonation of rocks, and, hence, in the abstraction of carbon 

 dioxide from the atmosphere, coupled with the greatest extension 

 of lime deposition and hence the setting free of carbon dioxide. 

 Thus there will be a tendency toward the amelioration of the cli- 

 matic conditions, and this will aid the great expansional evolution 

 of marine life favored by the broad expansion of the littoral belt 

 and the formation of numerous epicontinental seas. New marine 

 faunas and floras, often of a provincial type, will come into exis- 

 tence, which are likely to arise through parallel evolution from 

 closely related ancestors in the various provinces. Thus a wide- 

 spread basis for f aunal correlation will be inaugurated, such faunas 

 comprising not identical, but rather closely representative, species. 

 Pathways for extensive migration along the littoral belts of the 

 oceans also result, and these tend to produce widespread uniformity 

 of the littoral fauna of the oceans. Such periods of extensive trans- 

 gression of the sea and corresponding expansional evolution are 

 seen in the late Cambric and early Ordovicic, in the Middle, and 

 early Upper Ordovicic, in the early Siluric, Middle Devonic, early 



