48 PHYSIOGRAPHIC GEOLOGY. 



or those in which reef-making corals grow, so that the coral-reef seas and 

 Torrid Zone thus have the same limits. 



In the Pacific, the effects are no less striking than in the Atlantic, on the 

 ivest side of the American continent. Owing to the cold-latitude waters that 

 flow equatorward, the isotherm of 68° reaches the South American coast at 

 its west cape in latitude 4° S., and thus a tropical temperature is excluded 

 from nearly the whole of it. In the north Pacific, the cooling effect is much 

 less, because of the barrier to the arctic waters at the shallow Bering Strait ; 

 the isotherm terminates against the coast at 23°. But on the east border of 

 Asia and Australia, or the west side of the ocean, the width of the area 

 between the north and south isotherms of 68° is 65°, and the mean width in 

 the central Pacific is about 55°. 



The warm Gulf Stream extends its effects over the whole breadth of the 

 north Atlantic, even to Great Britain and Iceland and the polar seas, as 

 is indicated on the map by the long loops in the isotherm of 44° and 35°. 

 The warm waters extend to Spitzbergen near 82° N"., and to the west side of 

 Nova Zembla, where the absence of ice in summer is its effect ; and in favor- 

 able times it goes still farther east. Thus the heat of the tropics is made to 

 temper arctic climate. But by the time the waters have reached the polar 

 circle they have lost all tropical heat, and are warm only from contrast with 

 the mean temperature of the northern latitude. 



The effects of the polar waters along the east borders of North America 

 are strongly marked, because they there pass alongside of the warm Gulf 

 Stream from the south. The southward course near the continent of the 

 isotherm of 35° to the southern angle of Newfoundland, and the termina- 

 tion of the isotherms of 50°, 56°, and 62° at Cape Hatteras, are a conse- 

 quence of the Labrador waters. Down to this cape these cold waters cover 

 a cold belt inside of the Gulf Stream; but farther south they are excluded 

 by this stream. 



The polar waters are also felt, but to a less extent, on the borders of 

 northwest Europe. The effect is manifest also along the east Asiatic coast, 

 where, as the map shows, the isotherm of 35° extends down to 45° N., and 

 that of 68° even down to 15° N. 



Deep-water effects. — The great currents of the ocean have also deep- 

 water effects. They are, as has been shown, deep-water currents. The Gulf 

 Stream has a depth of 2500 to 1800 feet from the Plorida straits to Cape 

 Hatteras, and 1500 to 1000 north of the cape through the ocean ; and the 

 effects of the polar currents or movements are of all depths from the 

 surface to the bottom. Between the two systems of movements, that of 

 the tropical and that of the polar waters, the ocean derives its distribution 

 of heat. South of Cape Hatteras, the deeper waters of the Gulf Stream 

 give warmth to the bottom over a belt 50 to 75 miles wide ; and north of 

 this cape, the warm belt lies between the 65-fathom line on the west, where 

 stands the cold wall of the Labrador current, and the 200-fathom line on the 

 east ; giving a temperature of 53° to 47° (Verrill) to the bottom, while on 



