266 Proceedings of the Royal Physical Society. 
1 foot in 300 years as a fair average rate at which the growth 
of a volcano may be computed to have grown. No one who 
has carefully considered all the facts will regard this 
estimate as erring on the side of excess. We will therefore 
assume that the Skye Volcano, 8000 feet high over its central 
portion, grew at the rate of 1 foot in 300 years. Even at 
this comparatively rapid rate, and supposing that its growth 
went on without cessation, the volcano must have required 
2,400,000 years for its completion. It is well known, how- 
ever, that the growth of the volcanoes of that part was 
interrupted more than once or twice by long periods of 
quiescence, and we are therefore well within the truth in 
fixing the time at the rate just given. 
For the purpose in view in the present address, it will be 
taken for granted that the Skye Volcano began to erupt 
some time in the Oligocene Period—many would think we 
should be quite justified in assigning a later date even than 
that to the commencement of the great volcanic episode. 
Prior to the Oligocene Period there was formed on the 
Continent several thousands of feet of the marine Nummu- 
litic Limestone. Near Biarritz this rock is more than 3000 
feet in thickness. The evidence for the whole of this being 
of later date than the Chalk, and of older date than the 
Oligocene strata, is admitted on all hands. Prior, therefore, 
to the first outbreaks of the Skye Volcano, and posterior in 
date to the Upper Cretaceous Rocks, occurred the marine 
episode referred to. 
In dealing with the question of the rate at which rocks 
of sedimentary origin are being formed at the present day, 
we find ourselves at once face to face with the principal 
difficulty with which we meet in attempting to estimate the 
Age of the Earth. Of the rate of formation of terrigenous 
deposits we know as yet but very little. All one can say 
with tolerable certainty regarding the subject in general is 
that, as the greater part of the materials employed in their 
formation is carried seawards from the land by the agency of 
rivers, at a rate which is now (thanks to the labours of 
Tylor, Lyell, Croll, Geikie, and others) fairly well known, 
the rate of formation of a marine deposit of terrigenous 
