16 THE PRESIDENTIAL ADDRESS 
Drift, according to Joly and Holmes, is a cyclical phenomenon ; 
if present-day observations were to give a negative result they would 
not necessarily disprove it. 
The occurrence of recumbent rock-folds, and nearly horizontal 
slides or ‘nappes’ in mountain regions, gives positive proof that 
parts of the upper earth-crust have moved over the lower. In the 
North-west Highlands of Scotland a sliding of at least ten miles 
was proved by Peach and Horne, and in Scandinavia it amounts 
to sixty miles. For mountain packing as a whole the figures 
already given are far larger, while in Asia Argand has stated that 
packing of over 2,000 miles has occurred. ‘Thus, when all is said 
and done, movements on a colossal scale are established facts, and 
the question of the future is how far we shall accept the scheme of 
drift due to Wegener, or one or other of the modifications of it. 
It is for us to watch and test all the data under our own observation, 
feeling sure that we shall have to adapt to our own case Galileo’s 
words ‘ e pur si muove.’ 
Ever since it was realised that the inclination and folding of 
rocks must be attributed to lateral or tangential stress and not solely 
to uplift, shrinkage of the interior of the earth from its crust has 
been accepted as the prime mover, and whichever of the current 
theories we adopt we cannot deny the efficacy of so powerful a cause. 
The general course of events in the formation of a mountain range 
is fairly well known: the slow sinking of a downfold in the crust 
during long ages ; the filling of this with sediment pari passu with 
the sinking, and associated softening of the sub-crust due to accumu- 
lated heat; the oncoming of lateral pressure causing wave-like 
folds in the sediments and the base on which they rest ; the crushing 
of folds together till, like water waves, they bend over and break by 
over-driving from above or, it may be, under-driving from below ; 
fracture of the compressed folds and the travelling forward for great 
distances of slivers or ‘ nappes ’ or rock, generally of small relative 
thickness but of great length and breadth, and sliding upon floors 
of crushed rock; the outpouring and intrusion of igneous rocks, 
lubricating contacts and complicating the loading of the sediments ; 
metamorphism of many of the rocks by crystallisation at elevated 
temperatures and under stress, with the development of a new and 
elaborate system of planes of re-orientation and movement; and 
elevation of the whole, either independently or by thickening with 
compression and piling up to bring about a fresh equilibrium. 
Such a course of events would be brought about by lateral pressure 
developed during the consolidation phase of each of the thermal or 
magmatic cycles. At each period of their building, mountains have 
