Vice-President’s Address. 265 
the time the volcanic matter was being erupted, the dyke 
would have filled the valley instead of taking its present 
form. Hence, he points out, the valley must have been 
excavated since. Inferentially, therefore, any, or all, of the 
largest valleys that traverses the Highland plateaux may 
well have been formed since. No geologist fully cognisant 
of the vast and important physical and biological changes 
which can be shown to have taken place on the Continent 
and elsewhere since the close of the Miocene Period here 
referred to, can hesitate to regard the estimate of 16,000,000 
of years as being well within the mark. 
We have next to consider the time required for the build- 
ing up of a volcano of the dimensions above referred to. Sir 
Charles Lyell many years ago pointed out that the rate of 
growth of a modern volcano would appear to be very much 
slower than has commonly been supposed. There are 
several reasons why this is the case. (1) The eruptions are 
intermittent, and are often separated by long periods of repose, 
during which the ordinary agents of denudation tend to 
transport much of the newly formed volcanic material in the 
direction of the sea. (2) Violent explosive outbursts occur, 
perhaps several times, in the life-history of most volcanoes. 
These result in the shattering of much of the rock material 
previously accumulated, and aid in its transfer by the action 
of the winds to areas many hundred times the area of the 
parent masses. (3) The lavas resulting from effusive eruptions 
usually take the form of narrow streaks, which are radial in 
respect to their crateral starting-points; other lava streams 
following these from the same crater rarely or never overflow 
the ridge formed by their immediate predecessors, but 
usually follow the line of lowest ground, which generally 
lies in another direction. Hence the lavas flowing from a 
summit crater take the form of radial streaks. To com- 
plete the circuit of the cone may require, in the case of a 
volcano of the form of Etna or Hawaii, many thousands of 
years. Hence the average thickness deposited in a century, 
taking the volcano all round, must actually be very small. 
Professor Lloyd Morgan, writing upon the subject of 
Geological Time (Geological Magazine, 1878, p. 204), regards 
