512 C. SCHUCHERT PALEOGEOGRAPHY OF NORTH AMERICA 



and cold climates, it is evident that the life of these seas must have devel- 

 oped under great stress, and was therefore subject to more rapid evolution 

 than the life of the oceans. During times of transgression the marine 

 faunas not only increased numerically in individuals, but the latter 

 changed in form into new genera and species — "expansional evolution" — 

 while the "provincial faunas" of the early part of the invasions, through 

 intermigration, were more and more transformed into "cosmopolitan 

 faunas." In times of emergence the faunas were largely killed off, unre- 

 lated stocks were brought into direct struggle with one another for the 

 possession of food, the whole tendency being toward "restrictional evolu- 

 tion." 135 



Oceanic level. — No such condition as an oceanic level conforming to a 

 regular spheroid exists. Where the land-masses are large and elevated r 

 the oceanic level is attracted considerably higher than where these are 

 small and low — a fact long since appreciated by geodesists in computations 

 upon the form of the "geoid." At the present time the average height 

 of the continents is about 2,200 feet, and it is estimated that this mass 

 attracts the strand-line at least 300 feet higher than if the lands were 

 reduced to sealevel. In this way may be explained some of the smaller 

 overlaps during the early stages of times of emergence, as well as the 

 appearance of small synclinal seas on the inner sides of moving blocks of 

 land. As the high lands are rapidly eroded, these attracted seas and small 

 synclinal seas are likely to disappear, and the waters thus released aid in a. 

 general eustatic elevation of the strand-lines in the low land areas. 



Owing to volcanic activity throughout the geological ages, the waters on 

 the surface of the earth have increased to the present volume. This was 

 particularly so during the "formative eon" and the "extrusive eon," and 

 the additions must have been considerable since the earliest Proterozoic or 

 the " gradational eon." 136 Whatever the amount may have been since the 

 Proterozoic, it does not appear to have had visible effect in raising the 

 strand-line of any period. This extra water has been taken up appar- 

 ently by the constantly enlarging oceans and the fragmentation of conti- 

 nental areas and their vanishing into the deeps, such as Davis strait, Den- 

 mark strait, Norwegian sea, equatorial Atlantic (Gondwana), etcetera. 



As a vast amount of water has been added to the earth's surface during- 

 the geologic ages, and as nearly all of it has been added to the oceans, plus 

 some of the material of the horsts, may we not have in these constantly 





135 Chamberlin : A systematic source of evolution of provincial faunas. Journal of 

 Geology, Chicago, vol. 6, 1898, pp. 597-608. 



138 Chamberlin and Salisbury : Geology, vol. 2, 1906, p. 119. 



