256 DYNAMICAL GEOLOGY. 



7. Vaj'iations in the density of the earth'' s atmosjyhere. — The atmosphere 

 absorbs and retains heat, and the amount absorbed increases with its 

 density. In early geological time, the earth's atmosphere contained much 

 more carbonic acid and moisture than now, and hence it would have 

 absorbed more of the sun's rays as they passed through it. It has been 

 shown by Tyndall that the absorptive power of carbonic acid, under ordinary 

 atmospheric pressure, is 90 times greater than that of the atmosphere, and 

 that of moisture 30 to 70 times greater for non-luminous heat. Their 

 presence in the atmosphere would hence have greatly increased its power to 

 absorb and retain about the earth the sun's heat. " They would produce 

 little reduction in the amount of luminous sun-heat received, and would be 

 a formidable obstacle to non-luminous heat escaping by radiation from the 

 earth's surface into the cold of star-space " (Haughton, 1880). 



The earth's lower plains are warmer than its elevated regions, because of 

 the greater density of the air. The lowest places should thus have the 

 warmest climate ; and accordingly the basin of the Dead Sea, 1308 feet below 

 the sea level, has the heat of the torrid zone. 



8. Variations in oceanic currents. — The effect of the Atlantic tropical 

 current on the Arctic and north Atlantic climates has been elucidated by the 

 calculations of Mr. James Croll. His conclusion, based on the amount of 

 water that passes the Florida Strait (nearly agreeing with the latest esti- 

 mate), and the temperature of the water, is, that the amount of heat con- 

 veyed from the equatorial regions northward in the Atlantic by this stream 

 is equivalent to 77,479,650,000,000,000,000 foot-pounds of energy per day, 

 which is equal to all the heat received by 1,560,935 square miles at the 

 equator, and more heat than is conveyed by all the aerial currents ; and 

 that the stoppage or diversion of the current would diminish to this extent 

 the heat of the Arctic seas and north Atlantic. 



It has been supposed that the diversion of the Gulf Stream from the 

 north Atlantic may have taken place through the sinking of the region of 

 the Isthmus of Darien ; but there is no sufficient evidence that such a diver- 

 sion has happened since Mesozoic time. A more reasonable hypothesis is 

 that it may have been accomplished by a raising of the sea-bottom nearly 

 to the surface between Scandinavia, Great Britain, Iceland, and Greenland, 

 where the depth now is mostly less than 100 fathoms and nowhere exceeds 

 1000, and along one tract is not over 500 fathoms. The effect of such a 

 north Atlantic barrier would be to confine the Atlantic tropical current to 

 the north Atlantic, and thereby to increase the temperature and amount 

 of evaporation of that ocean. It would reduce the northern part of the 

 stream to the southeast branch, and might diminish its volume; but, in 

 view of the form of the south Atlantic depression and its position with 

 reference to the north Atlantic, the warm stream could not fail to continue 

 its flow. 



Again, the Arctic region may formerly have had its climate moderated 

 by receiving the Pacific tropical current, through a submergence about 



