METHODS OF PALEOGEOGRAPHY 443 



the Arctic currents that flow south along this region. Impinging against 

 the outer edge of the continental shelf in the deeper water occurs the 

 warm Gulf stream, and yet the two faunas are quite distinct to within 

 about 10 miles of each other. All along the present shores, where there 

 are currents of cold and warm water, it is the rule to find distinct faunas 

 on each side of prominent land promontories. Furthermore, many of the 

 cold-water species of the northern Atlantic follow the cold water south of 

 cape Hatteras into the deeps beneath the Gulf stream. 



While the truth of these statements is not to be denied, yet in dealing 

 with existing life as the basis for interpreting geologic faunas one must 

 not lose sight of the important fact that the marine waters of today are 

 those of a glacial climate. During most of geologic time the temperature 

 of the ocean was far more even than at present, with no such variations as 

 now occur in the northern Atlantic from 72 degrees Fahrenheit at the 

 surface to 33 degrees Fahrenheit or even less in the abyss. During the 

 Ordovicic, Siluric, and Devonic there were only slight differences in tem- 

 perature, and these variations were no doubt due to latitude. That during 

 these times the marine waters had a nearly equable temperature is seen in 

 the very similar Mohawkian faunas of Tennessee, ~Ne\v Jersey, Minnesota, 

 and Baffin Land; reef corals of Devonic age occur in abundance not only 

 in Kentucky and Indiana, but almost equally so in Alaska, while certain 

 of the Siluric faunas of the interior region are undoubtedly derived from 

 those of northern Europe, which migrated to America by way of north 

 Greenland. Finally, attention may be directed to the fact that in Miocene 

 times magnolias flourished in Greenland. 



The region of the strait of Gibraltar is a good example of an almost 

 complete land barrier that is ineffectual as a faunal barrier because of a 

 small opening in it. The Mediterranean entrance into the Atlantic is 

 only 9 miles in width, yet hundreds of warm, shallow-water species have 

 spread north along the coasts of Spain, Portugal, France, and even to the 

 south shore of England. Some of the species have distributed themselves 

 along this shoreline fully 1,500 miles, measured in a straight line. On the 

 other hand, the deep-sea faunas on each side of this submerged barrier 

 at Gibraltar do not spread, owing to the great difference in temperature. 

 From this it is seen that however small an opening may exist between two 

 bodies of water with nearly the same temperature (Atlantic, 72 degrees; 

 Mediterranean, 75 degrees), the two faunas will intermigrate in spite of 

 the further fact in the present instance that the Atlantic is less saline and 

 becomes progressively cooler toward England. 



It can be said with certainty that in marine continuous waters, either 

 warm or cold, but of fairly equable temperature throughout the year, the 



XLI — Bull. Geol. Soc. Am., Vol. 20, 1908 



