/. Clifton Ward — On Geological Time. 11 



the place of oceanic islands, to have been commenced before the 

 Glacial Epoch. Let the accompanying woodcut represent the out- 

 line of the great Pacific Continent ; the line A B is the sea level soon 

 after the commencement of the Glacial Epoch, say 240,000 years 

 ago ; suppose the land to be regularly sinking at the rate of one foot 

 per century, in 160,000 years (the close of the Glacial Epoch) the 

 sea-level would be represented by the line D, the distance x y 

 being 1600 feet in vertical height. During this interval, however 

 (according to our supposition), the reefs 1, 2, 3 north of the equator 

 would have ceased to grow upwards, while the reefs 4, 5, 6 south of 

 it would have" continued their upward growth ; after this period of 

 160,000 years the reef-builders would be able to recommence work 

 north of the equator, and No. 4 reef might gradually extend itself 

 further north (in the direction of the arrow) until the Post-glacial 

 reef No. 7 would commence and finally form with No. 4 one great 



North EairATOR South 



Ideal representation of Old Pacific Continent, to illustrate the theory of the gradual subsidence ' 

 of the land and the formation of Coral-Keefs. 



encircling reef; but owing to the great depth of ocean which would 

 now separate G from H no reef-builders could find their way to the 

 island G, which would, although other conditions were favourable, 

 be without a reef of any kind ; and in some instances it might be 

 the case that the reef 4 would not be able on account of the deep 

 water to wrap round the north side of the sinking island, and in this 

 way some of the strange forms of reefs may be occasioned. The 

 line E P represents the sea-level as it now is after a lapse of 80,000 

 years, the distance x^y^ representing 800 feet in vertical height. We 

 should thus expect to find in the Coral regions north of the equator 

 some islands without reefs of any kind, others with encircling reefs, or 

 atolls of no very great thickness, and therefore small size.^ This 

 view is somewhat confirmed by the fact that the atolls of the Pacific 

 ''diminish towards the equator and disappear mostly north of it;"- 

 and I believe I am correct in stating that the thickest reefs, so far as 

 can be ascertained with any accuracy, occur mostly south of the 

 equator. Dana estimates the whole subsidence in the Pacific area 

 to be not less than 6000 feet, and adds,^ " It is probable that this 

 sinking began in the Post-tertiary period." Now, since it seems 

 likely that the general rate of subsidence cannot be much more than 

 a foot per century, in order that the corals may keep pace with it, 

 the sinking of this 6000 feet of land would take 600,000 years, 



1 Of course the thicker a reef is the larger mil be the area which it includes. If 

 Darwin's chart of Coral-Eocks be referred to, it will there be seen that north of 10^ 

 north lat., there are but one or two small atolls, while in the southern hemisphere 

 they extend in mass as far as the parallel of 25°. 



2 Dana's Manual of Geology, p. 587. 



