i873-J Coral Reefs and the Glacial Period. 177 



Example : The reefs of Florida may have taken 70,000 years 

 in formation — therefore the glacial climate, with its ice-sheet 

 extending south into 39 north latitude, cannot have ceased 

 less than 70,000 years ago, because these reefs have every 

 appearance of having been uninterruptedly formed, and not 

 of being partly pre- glacial and partly post-glacial.* Again, 

 the South Pacific atolls represent a sinking of 6000 feet ; 

 this at 1 foot per century, without allowing for intervals, 

 would give 600,000 years for their formation, which from 

 the very nature of atolls must have been continuous — there- 

 fore a glacial climate, with an ice-sheet on the continental 

 lands extending north into 37° south latitude, could not 

 probably have existed in the southern hemisphere within 

 that period. 



5. It must, however, be remembered that one of the con- 

 ditions for the formation of atolls is the existence of an 

 area of slow subsidence, which condition may not have 

 occurred in the northern hemisphere within the 68° isotherm. 



6. If an extreme glacial climate occurred latest in the 

 southern hemisphere, and coincidently with it an extreme 

 hot climate in the north, we might expect to find the oldest 

 coal-reefs north of the equator. 



7. An extreme glacial climate prevailing in both hemi- 

 spheres simultaneously would restrict the coral reefs to very 

 narrow limits on either side of the equator, unless, owing to 

 a less amount of land in the southern hemisphere, the ice- 

 sheet there should not be so continuous and extensive, in 

 which case irregularities in their distribution might occur. 

 On this supposition, the age of the most southerly South 

 Pacific atolls would probably indicate the least time that 

 could have elapsed since the ice-sheet disappeared, since 

 atolls necessitate a continuous act of formation, and they 

 could not be formed partly in pre-glacial and partly in post- 

 glacial times. 



8. If the glacial climate prevailed last in the northern 

 hemisphere, we should expect to find north-temperate forms 

 of life more numerous than south-temperate on equatorial 

 highlands ; if the southern hemisphere was the most 

 recently glaciated, the south-temperate forms would be 

 more abundant on the equatorial highlands than the north- 

 temperate ; if both hemispheres were glaciated at the same 

 time there would be about an equal mingling of north- and 

 south -temperate forms about the equator. 



9. The facts of the case are — (1) That the largest 



* I believe I am right in saying that there is no evidence of a long break in 

 their formation. 



VOL. III. (N.S.) 2 A 



