318 MAN AND THE GLACIAL PERIOD. 



is adduced by Mr. Upham and others as one of the 

 important probable causes of the Glacial period, namely, 

 the subsidence of the Isthmus of Panama and the adja- 

 cent narrow neck of land connecting North with South 

 America. It will be seen at a glance that a subsidence 

 sufficient to allow the northwest current of warm water, 

 pushed by the trade-winds along the northeast shore of 

 South America, to pass into the Pacific Ocean, instead of 

 into the Gulf of Mexico, would be a cause sufficient to 

 produce the most far-reaching results ; it would rob the 

 North Atlantic of the immense amount of heat and moist- 

 ure now distributed over it by the Gulf Stream, and would 

 add an equal amount to the northern Pacific Ocean, and 

 modify to an unknown extent the distribution of heat and 

 moisture over the lands of the northern hemisphere. 



The supposition that a subsidence of the Isthmus of 

 Panama was among the contributing causes of the Glacial 

 period has been often made, but without any positive 

 proof of such subsidence. From evidence which has re- 

 cently come to light, however, it is certain that there has 

 actually been considerable subsidence there in late Ter- 

 tiary if not in post-Tertiary times. This evidence is fur- 

 nished by Dr. G. A. Maack and Mr. William M. Gabb in 

 their report to the United States Government in 1874 

 upon the explorations for a ship-canal across the isthmus, 

 and consists of numerous fossils belonging to existing 

 species which are found at an elevation of 150 feet above 

 tide. As the dividing ridge is more than 700 feet above 

 tide, this does not positively prove the point, but so much 

 demonstrated subsidence makes it easy to believe, in the 

 absence of contradictory evidence, that there was more, 

 and that the isthmus was sufficiently submerged to permit 

 a considerable portion of the warm equatorial current 

 which now passes northward from the Caribbean Sea and 

 the Gulf of Mexico to pass into the Pacific Ocean. 



An obvious objection to the theory of a late Tertiary 



