172 Coral Reefs and the Glacial Period. [April, 



that continent down to the parallel of 39 , while the greater 

 part of Northern Europe was similarly ice-clad. 



The present southern limit of perpetually frozen ground 

 in the northern hemisphere is, for a great part of its course, 

 between the parallels of 55 and 6o°, though, from the 

 southern point of Greenland to the eastern part of Russia, 

 it runs up to a latitude of 70 . The northern isotherm of 

 68° roughly corresponds to a latitude of from 30 to 35 , so 

 that now there is an average of 25° of latitude between the 

 southern limit of frozen ground and the northern limit of 

 reef-builders, though, at two points, they approach one 

 another within about 15 . If the former be extended south- 

 wards as far as the parallel of 40 , are we justified in con- 

 cluding that the latter would be thrust southwards in a 

 proportional degree, that is, to a parallel of from 5 to io° ? 

 If so, it is clear that the Equator of Heat, instead of being, 

 for the greater part of its course, north of the equator, as 

 now, might be considerably south of it, even supposing the 

 climate of the southern hemisphere to be no warmer than 

 at present. But if, as we are supposing, the southern 

 hemisphere was under a very hot climate, the equator of heat 

 might be still farther removed from the geographical equator. 



Under this condition of things, it would seem certain 

 that there could be but a small range of reefs north of the 

 equator, but that they might extend farther south than at 

 present. Supposing the climate of both hemispheres slowly 

 to approximate to that which now prevails, it is evident 

 that the oldest reefs would be found south of the equator. 

 Now the atolls undoubtedly furnish the evidence of greatest 

 antiquity, since their formation — on Darwin's view of their 

 origin — clearly shows a very gradual sinking of land 

 throughout immense periods of time. Hence we should 

 expect to find the greatest number of atolls south of the 

 equator, and the reefs north of it to belong mostly to the 

 classes of fringing and barrier reefs, which is indeed found 

 to be the case. Moreover, could we hit upon some sure 

 average rate of the growth of reefs, and know the exact 

 relation which such rate of growth bears to the rate of sub- 

 sidence, or otherwise, of the land, we should have, in these 

 more northern reefs, some indication of the time that has 

 elapsed since the close of the glacial period in the northern 

 hemisphere. 



In an article in the " Geological Magazine " for January, 

 1869, I suggested that the present distribution of coral reefs 

 seemed to show that those south of the equator were of 

 much greater age than those north of it, that the sinking of 

 the supposed old Pacific continent, perhaps, commenced 



