270 A. E. Ortmann — Climatic zones in Jurassic times. 



be more profitable to examine closely the differences of the 

 facies of this time, especially the question, how the different 

 facies are deposited, and how the different groups of animals 

 are influenced by the facies. There is no doubt that for this 

 purpose we have to compare the fossil deposits with those 

 formed in the recent seas. This latter method is the way out- 

 lined by J. Walther in his work : " Einleitung in die Geologie 

 als historische Wissenschaft," and it would be very advan- 

 tageous to direct more attention to this subject in subsequent 

 geological studies. 



Princeton, N. J., January, 1896. 



pre-Tertiary times. But such chariges and differences of the ocean-waters were 

 only slight ones, not differing from those present nowadays within the tropical 

 seas, and not influencing the distribution of animals. On the other hand, the 

 air-temperatures of the continents changed even in pre-Tertiary times in proportion 

 to the size of the continental masses to the elevation and to the geographical 

 latitude. Perhaps they did so in a lesser degree than at present, but this differ- 

 ence of the oceanic and the continental conditions of temperatures was almost as 

 striking as it is nowadays between the temperatures of the recent tropical seas 

 and those of the high mountain-chains and table-lands of the tropical parts of the 

 recent continents. 



