4 Proceedings of the Royal Physical Society. 
men at the present day hold the doctrine of what is termed 
“the permanence of the ocean-basins,” a doctrine which 
implies a greater or less amount of stability in the position 
of the great continental areas. In considering this theory, 
it may be as well to remind you of what the present state 
of things is. Were the earth’s surface smooth, there is 
enough water in the existing seas to give rise to a con- 
tinuous ocean of about half a mile in depth. As a matter 
of fact, the terrestrial surface is uneven, some areas being 
nearer to the earth’s centre than others, thus giving rise 
respectively to the ocean-basins and the continents. The 
dry lands cover 28 per cent. of the earth’s surface, 
and have a mean elevation of 2250 feet above the sea- 
level. The ocean covers 72 per cent. of the earth’s 
surface, and has a mean depth of 12,000 feet. Hence 
the elevated areas are not only of relatively small extent, 
but they are also of relatively small height, as compared 
with the area and depth of the oceanic depressions. Again, 
the continents and continental islands are generally margined 
by a broad zone of shallow water, showing that they are 
merely the exposed portions of great terrestrial platforms, 
the edges of which are covered by the sea. The true 
border of the proper ocean is the edge of the submarine 
plateaux upon which the continents stand. At the edge of 
this the water may be from 100 to 300 fathoms in depth, 
and then commences a comparatively rapid slope, which 
leads to the vast regions forming the floor of the deep sea. 
The floor of the deep sea, lastly, is not absolutely flat, but is 
undulated, and its depth below the surface varies from 1000 
to 4600 fathoms. 
Stated in its briefest form, the doctrine of the “ per- 
manence of the ocean-basins ” holds that though our existing 
dry lands must have been repeatedly submerged below the 
level of the sea, such submergence was never to great 
depths, and was, in any given period, of an altogether 
partial character. It is held, therefore, that our existing 
continents have been built up bit by bit, by partial sub- 
mergence beneath the sea at different places and at different 
times; but that, on the whole, they have retained their 
