Vice-President’s Address. 271 
lime-salts carried seaward annually by rivers is left in the 
solid form over one-third of the entire ocean area, and that 
the supply from the rivers is largely augmented by supplies 
derived from the possible sources mentioned, we still leave a 
large margin for the deposition of carbonate of lime from 
other—entirely unknown—sources, if, instead of 1 foot in 
41,985 years, we regard it as accumulating now, and as 
having accumulated on the whole in the past, at 1 foot in 
25,000 years. 
If we estimate the thickness of the foraminiferal Num- 
mulitic Limestone at only 3000 feet, this alone would 
require an interval of no less than 75,000,000 of years for 
its completion. 
It is right to observe at this point that if there is any 
truth in the speculation that the atmosphere at past periods 
of the Earth’s history contained even a slightly higher per- 
centage of carbon-dioxide than it has now, the formation of 
limestone must have proceeded at even a slower rate than 
this. Also, if the ocean (as is supposed by many persons) 
formerly had a larger area relative to the land than it has 
now, the formation of limestone at the bottom of the sea in 
past times, so far from being more rapid than it is now, must 
have gone on at even a slower rate still. I shall therefore 
assume throughout this paper that 1 foot in 25,000 years is a 
fair rate to allow for the formation of marine limestones other 
than coral-reefs, with which geologists are but little concerned. 
Summarising these figures, we have therefore for the dura- 
tion of the Tertiary Period :—Time since the Glacial Period, 
20,000 years; time for the excavation of the present land 
surface of the Western Island volcanic areas, 16,000,000 
years; growth of the Skye Volcano, 2,400,000 years; time 
required for the formation of the Nummulitic Limestone, 
75,000,000—total since the commencement of the Tertiary 
Period, 93,420,000 years. 
Ninety-three millions of years may seem to those who are 
not fully acquainted with the succession of events that has 
taken place since the commencement of the Tertiary Period 
to be too extravagant an estimate to be for a moment 
seriously considered. With both geologists and biologists, 
