i873«] Coral Reefs and the Glacial Period. 171 



much of actual reefs, as by their disintegration and the 

 widespread deposition of coralline sediment. 



Geology made yet another great advance when Agassiz, 

 bringing his knowledge of existing glaciers to bear upon 

 certain phenomena in Scotland and England, showed how 

 certainly our now temperate climate was once an arctic one, 

 and that the diluvial phenomena were for the most part 

 easily explained, on the supposition of the existence of a 

 former glacial period. 



Of late, the possibility of determining the time that has 

 passed away since that period of extreme cold has animated 

 the hopes of geologists, and the physicist and astronomer 

 combined have brought their knowledge to bear upon the 

 question. The result, so far, is known to all through the 

 various papers of Mr. Croll. But since truth will stand all 

 shocks, and show itself more truth-like after each attack, 

 it should be the aim of geologists to test all theories put 

 forward to explain series of facts in every possible way ; 

 and more especially when those theories are supported by 

 mathematical arguments and reasoning is it incumbent upon 

 the students of nature to see well to the ground-work upon 

 which the mathematician builds his indisputable structure. 



It is well known that, according to Mr. Croll's explana- 

 tion of the cause of the glacial period, those agents producing 

 an extremely cold climate over the greater part of the 

 northern hemisphere would give rise to a proportionally 

 hot one in the southern. 



Scientific observation in the southern hemisphere is now 

 bringing before us the fact that, at no very distant period, 

 an extreme glacial climate prevailed there also, and, more- 

 over, that the relics of a former great ice-sheet in the 

 southern hemisphere seem as fresh and of equal value to 

 those of the northern ice-sheet. 



Coral reefs are, at the present day, confined within the 

 isotherms of 68°. In proportion as the ice-sheets in either 

 hemisphere are extended into lower latitudes, so must the 

 isotherms of 68° approach the equator, and the coral reef 

 zone become restricted. Let us see what would be the 

 probable result upon the distribution of coral reefs, 1st, of 

 an extended ice-sheet in the northern hemisphere, and, at 

 the same time, an increase of heat in the southern ; 2nd, of 

 an extended ice-sheet in the southern hemisphere, and a pro- 

 portional increase of heat in the northern ; 3rd, of greatly 

 extended ice-sheet in both hemispheres at the same period. 



1. The study of North American geology shows that, 

 during the glacial period, an ice-sheet completely enveloped 



