/. Clifton Ward — On Geological Time. 9 



the least wliicli could have elapsed since the deposition of the Glacial 

 Drift. 



Under the second class of facts, which is, perhaps, less reliable, 

 we may reason thus : — 



1st. Elevation. — All observations tend to show that the elevation 

 or depression of the earth's surface takes place very gradually, 

 and extends with tolerable constancy over long periods (this the 

 thick coral-reefs of the Pacific seems to prove) ; supposing we are 

 able to conclude generally that this movement averages one foot per 

 century over many parts of the earth, then marine Glacial deposits 

 now 1000 feet above the sea would show that they were formed not 

 less than 100,000 years ago.' 



2nd. Subsidence. — The case of the Coral-reef in "building up" 

 may also be taken as an example of subsidence within a given period. 

 Now taking some actual examples let us see whether they, even 

 roughly, lend any support to the results arrived at by Mr. Croll. 



The Peninsula of Florida is made up of some ten coral-reefs one 

 within the other. The whole Peninsula, being only 16 or 17 feet 

 above the sea-level, is formed of the same species of coral, that 

 species being still at work in forming the outermost reef, between 

 which and the Island of Cuba the Gulf Stream flows. Professor 

 Huxley, calling each reef 70 feet thick, and taking the upward 

 growth as one foot per century, has estimated the period for the 

 growth of one reef as 7000, and for the whole Peninsula as 70,000 

 years, supposing the conditions necessary for a constant increase in 

 height of the reef to be favourable. 



Dr. Dana, in his excellent Manual, divides the Post-Tertiary 

 Period into the following Epochs : — Glacial Epoch, Champlain 

 Epoch, Terrace Epoch. 



The Glacial Epoch includes the true Glacial Drift, a deposit due 

 to a vast spread of land-ice reaching down to the latitude of 39°, the 

 epoch being one, Dr. Dana considers, of high latitude elevation. 



A partial subsidence of the country brought in the Champlain 

 Epoch, softening the climate, melting the vast ice-sheet, swelling 

 the rivers, and causing great thicknesses of fresh-water deposits to 

 be formed along their course and around the large lakes. The shells 

 in these deposits indicate a temperature very similar to the present. 

 During the Terrace Epoch the land was again gently elevated, the 

 elevation being greater in high latitudes — as much as 1000 feet in the 

 extreme north — and lessening southwards, where probably the up- 

 ward movement was completely lost ; then it was that the Champ- 

 lain deposits were carved into terraces, and sea beaches left high 

 and dry. 



But to return to the Glacial Epoch : if North America was clad 

 with one universal sheet of ice down to the parallel of 39°, it would cer- 

 tainly have been impossible for coral-animals to have been reef -building 



^ Example — The marine shells at an elevation of 1400 feet at Moel Trji'aen, near 

 Bangor ; for even if the hypothesis of a 600 feet fall in the sea-level be ti'ue, the land 

 must have been raised 800 feet, giving a period of 80,000 years at the rate of one foot 

 per century. 



