438 TRANSACTIONS OF SECTION A. 
superposed upon an elevation represented by the other two harmonics, and the 
same is true of the deep parts of the Indian Ocean which border the shores of 
Africa and Asia from Madagascar to Burmah. The deep parts of the Pacific that 
border the western coast of America from Alaska to Chile are regions in which 
an elevation represented by the third harmonic is superposed upon a depression 
represented by the other harmonics. These observations suggest that in the 
greater part of the Atlantic and the northern and western parts of the Indian 
Ocean the direction of secular change may have been that of an advance of the 
ocean to encroach upon the continental region, while in the Pacific Ocean on the 
American side the direction of secular change may have been that of a retreat of 
the ocean, permitting an extension of the continental region. This difference 
would lead us to expect different types of coast in the two regions, and such a 
difference has been observed. Whereas in the Atlantic region, with few excep- 
tions, the coast cuts across the directions of the mountain chains, in the Pacific 
region on the American side the coast generally corresponds in direction with the 
neighbouring mountain chains of the continent. The deep parts of the Pacific 
which are nearest to the Asiatic coast from Kamchatka to Siam are regions where 
a moderate depression represented by the third harmonic is superposed upon a 
moderate elevation represented by the other harmonics, These shores of the 
Pacific are distinguished by the wide margin which separates the deep ocean from 
the coast of the continent. It might perhaps be desirable to recognise in this 
region a type of coast differing from the two main types associated with the 
Atlantic and the American side of the Pacific. The analysis does not represent 
South Africa or the southern parts of South America sufficiently well to warrant 
usin expecting these regions to exhibit one type rather than the other; but the way in 
which Australia is represented, as an elevation of the third degree superposed upon 
a depression of the first, suggests that the coasts of Australia, and especially the 
eastern coast where the elevation in question is greater, should be of the same 
type as the American shores of the Pacitic; and it is the fact that the mountain 
chains of Queensland and New South Wales run parallel to the neighbouring 
coasts. ‘There seems therefore to be much evidence to support the view that the 
direction of secular change bas been that of diminishing the prominence of the 
inequalities of the first and second degrees in comparison with those of the third 
degree. The process by which such changes would be brought about would be of 
the nature of relief of strain, expressing itself in occasional fractures of no very 
great magnitude; and such fractures would be manifested at the surface as earth- 
quakes, Seismic and volcanic activities constitute the mechanism of the process 
of change. These activities are spasmodic and irregular, but the effect of them 
is cumulative. For this reason they tend in the course of ages to transform the 
shape of the earth from one definite type to another. The diminishing speed of 
the earth’s rotation is anotber cause of change which appears to produce an 
alternating rather than a cumulative effect. On the one hand it tends to diminish 
that tendency, which we noted above, to draw the waters of the ocean towards 
equatorial regions; on the other band it must result in an actual reduction of the 
equatorial protuberance of the earth’s figure. This reduction can only be effected 
by seismic activity expressed by subsidences in equatorial regions. The effect 
which would in this way be produced in the distribution of continent and ocean 
would appear to be that there would be long periods in which the ocean would 
tend to advance towards the Arctic and Antarctic regions, interrupted by shorter 
periods in which it would tend to retreat towards the neighbourhood of the 
equator. 
The theory which I have tried to explain is a tentative one, and further 
investigation may prove it to be untenable; but it is to its credit that, besides 
tracing to dynamical causes the existing distribution of continent and ocean, it 
offers an explanation of the difference between the Atlantic and Pacific types of 
coast, it gives indications of a possible account of those alternations of sea and 
land which first led to the study of geology, and it suggests an origin for Charles 
Darwin’s unknown force, the operation of which is slow and intermittent, but 
irresistible, , 
