THE PRESIDENTIAL ADDRESS 17 
arisen along lines of weakness in the crust, especially coast lines and 
the steep slopes marking the limits between continents and ocean 
basins. This is consistent with Joly’s theory that the thrust of 
ocean beds against land margins is the cause. 
But the advocates of continental drift point to the siting of ranges 
across the paths along which the drifting movement is supposed 
to have occurred, and they consider that the moving masses are 
responsible ; and indeed that the ridging and packing of the crust 
has in the end checked and stopped the movement. They note 
that the great western ranges of America occur in the path of any 
western drift of that continent, the Himalayas in the course of the 
postulated movement of India, the East Indies in front of Australia ; 
and that the Alpine ranges of Europe may be linked with the crushing 
of Africa towards the north. 
The ‘ nappes ’ of rock, cut off from their origin and sliding for 
dozens of miles, are a constant source of wonder to all who have 
considered the mechanics of mountain formation. They are so 
thin as compared with their great length and breadth, that it seems 
impossible to imagine them moved by any force other than one 
which would make itself felt throughout their every particle. Such 
a force is gravitation, and it is of interest that some Alpine geologists 
and Dr. Harold Jeffreys have used it in explanation of them. 
Professor Daly has also adopted gravitation on an even greater scale 
in his theory of continental sliding : and one cannot fail to notice 
the increasing use of the term ‘ crust-creep ’ by those working on 
earth-movement. 
Is there no other force, comparable in its method of action to 
gravitation, but capable of producing movement of the earth-crust 
in a direction other than downhill ? Is it not possible, for instance, 
that the tidal influence of the moon and sun, which is producing so 
much distortion of the solid earth that the ocean tides are less than 
they would be otherwise, and, dragging always in one direction is 
slowing down the earth’s rotation, may exert permanent distorting 
influence on the solid earth itself ? May it not be that such a stress, 
if not sufficiently powerful to produce the greater displacements 
of continental drift and mountain-building, may yet take advantage 
of structures of weakness produced by other causes, and itself 
contribute to the formation of nappes and to other movements of 
a nature at present unexplained ? 
Our knowledge of geology has been gained by the survey of the 
rocks, the study of their structures, and the delineation of both 
