44 PHYSIOGRAPHIC GEOLOGY. 



as » n ; it would be thrown south on the west side of the ocean by the warmth 

 of the torrid zone, and north on the east side by the cooling influence derived 

 from its flow in the cold-temperate zone. But if the current, instead of being 

 equable throughout the area, were mainly apparent near the continents (as is 

 actually the fact), then the isothermal line should take a long bend near the 

 coasts, as in the line A' r' r r r r A, or a shorter bend A' s *', according to the 

 nature of the current. This form of the isothermal line of 68° on the chart, 

 hence, indicates the existence of the circuit movement in the ocean, and also 

 some of its characteristics.* 



45. The following are some of the uses of this subject to the 

 geologist : — 



1. A wide difference is noted between the water-temperatures of 

 the opposite sides of an ocean. The regions named temperate and 

 sub-temperate occupy the most of the Mediterranean Sea, and the 

 Spanish and part of the African coast, on the European side, and 

 yet have no existence on the American, owing to the meeting at 

 Cape Hatteras of the cold northern waters with the warm southern. 

 Compare also other oceans and coasts on -the map. 



2. Consequently, the marine productions of coasts or seas in the 

 same latitudes differ widely. Corals grow at the Bermudas in 34° 

 N., where the warmth of the Gulf Stream reaches, and, at the same 

 time, are excluded from the Galapagos under the equator. Other 

 examples of the same principle are obvious on the chart. 



3. The west side of an ocean (as in the northern hemisphere) 

 feels most the cold northerly currents when the continent extends 

 into the polar latitudes ; but the east side (as in the southern hemi- 

 sphere), if the continent stops short of those latitudes. There is 

 hence in the present age a striking difference between the northern 

 and southern hemispheres. 



4. Changes of level in the lands of the globe have caused changes 

 of climates in the ancient world. 



5. Knowing the temperature limiting the coral-reefs of the pre- 

 sent era, or any species of plants or animals, the geologist has a 

 gauge for comparing the present distribution of temperature and 

 life with the past. 



5. ATMOSPHERIC CURRENTS AND TEMPERATURE. 



46. General System. — The system of atmospheric movement has 

 a general parallelism with that of the ocean. In the tropics the 



||| flow is from the east, constituting what are called the trades; in high- 



temperate latitudes it is from the west ; and the two pass into one 



* See paper by the author, in Amer. Jour. Sci. [2] xxvi. 231. 



