10 /. Clifton Ward — On Geological Time, 



only 9° farther south. From this one would argue either that the reef- 

 builders of Florida began their work before the Glacial Epoch, left 

 it off as the climate became cold, and resumed it at the close of that 

 period, or that the whole structure was of Post-glacial age. If the 

 former was the case, it would seem probable that there might be 

 some indication of it, either in a change of species, or in some more 

 or less marked break in the structure of the Peninsula. I believe 

 there is no such indication ; therefore the evidence would lead one to 

 conclude the whole structure to be Post-glacial, and we have already 

 seen that probably the work cannot have taken less than 70,000 years. 

 But if the cold of the Glacial Epoch could arrest the labour of 

 the reef-builders at this particular part, it probably had a similar 

 effect upon other parts likewise. The sheet of ice upon Greenland 

 is acknowledged to have a great effect in cooling our climate at the 

 present time ; its removal would have, as Mr. Croll says,' quite a 

 " magical effect upon the entire northern hemisphere : " imagine, 

 then, the power that a sheet of ice occupying the greater part of the 

 northern hemisphere would have in cooling those portions of the 

 globe immediately south of it. There is, however, another point to 

 be considered, viz., that according to Mr. Croll's theory the causes 

 which would in the northern hemisphere produce an extremely 

 cold climate, would in the southern produce a proportionally 

 hot one ; the hot southern hemisphere, however, would have its 

 climate more influenced by the cold of the northern, than the northern 

 would have its cold climate affected by the heat of the southern ; — 

 therefore the heat would have to retreat as it were before the cold, 

 and the probability is that instead of the hottest part of our globe 

 being situated around the equator, it would lie considerably south of 

 it. In this way the two isotherms of 68°, which are the limits of 

 reef-builders at the present day, would be carried considerably south 

 of their present position, and Coral-reefs would then probably only 

 be in process of formation south of the equator. But do not Coral- 

 islands in mid-ocean, with deep water all around, indicate a con- 

 tinuity of upward growth ? For suppose an island in the Pacific 

 north of the equator, rising 1000 feet above the sea, to be encircled 

 by a reef ; if the land sink at the same rate as the corals can build, 

 say one foot per century, the reef will grow upwards, the island 

 lessening in diameter. But now suppose a Glacial climate in the 

 northern hemisphere to come on and seriously to affect the reef- 

 builders, they must of necessity die off, migration being impossible 

 by reason of the deep surrounding sea. The subsidence of the land, 

 however, would probably continue, and when the cold was over and 

 the reef -builders might have found it possible to continue their work, 

 there would be no means for them to gain the shores, and the island 

 would go down without leaving an atoll behiad as tombstone to mark 

 the spot of its watery grave. 



Supposing, then, the Glacial climate had a marked effect upon 

 the reef-builders north of the equator, it would be impossible for 

 any of the reefs we now see in that region, surrounding or taking 

 1 Article in Philosophical Magazine for November, 1868. 



