42 



PHYSIOGRAPHIC GEOLOGY. 



current, in cooling the waters on the coast of America, is also well ex- 

 hibited in the bending southward near the coast of all the lines from 

 68° to 35°. The polar current is even more strongly marked in the same 



Fig. 31. 



way on the Asiatic coast. The lines from 

 74° to 35° have long flexures southward 

 adjoining the coast, and the line of 68° 

 comes down to within fifteen degrees 

 of the equator. These waters pass south- 

 ward mostly as a submarine current, and 

 are felt in the East Indies, making a 

 southward bend in the heat equator. 



In figure 31, the elliptical line (A'B' AB) represents the course of the current in an 

 ocean south of the equator (E Q). If now the movement in the circuit were equable, 

 an isothermal line, as that of 68°, would extend obliquely across, as n 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), 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 s', 

 according to the nature of the current. This form of the isothermal line of 68° on the 

 chart, indicates the existence of the circuit movement in the ocean, and also some of its 

 characteristics. 1 



The following are some of the uses of this subject to the geolo- 

 gist : — 



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 hemisphere), 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. 



1 See paper by the author, in Amer. Jour. Sci-, II. xxvi. 231. 





