540 



SCIENCE 



[N. S. Vol. XXVI. No. 669 



pression represented by the other har- 

 monics. 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 exceptions, 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 neighboring 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 recognize 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 suffi- 

 ciently . well to warrant us in expecting 

 these regions to exhibit one type rather 

 than the other ; but the way in which Aus- 

 tralia is represented, as an elevation of the 

 third degree superposed upon a depression 

 of the first, suggests that the coasts of Aus- 

 tralia, and especially the eastern coastwhere 

 the elevation in question is greater, should 

 be of the same type as the American shores 



of the Pacific; and it is the fact that the 

 mountain chains of Queensland and New 

 South Wales run parallel to the neighbor- 

 ing coasts. There seems therefore to be 

 much evidence to support the view that the 

 direction of secular change has been that 

 of diminishing the prominence of the in- 

 equalities 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 occasienal 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 ciimula- 

 tive. 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 rota- 

 tion is another cause of change which ap- 

 pears to produce an alteimating 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 hand it must result in an actual re- 

 duction of the equatorial protuberance of 

 the earth's figure. This reduction can 

 only be effected by seismic activity ex- 

 pressed by subsidences in equatorial re- 

 gions. 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 neighborhood of the 

 equator. 



The theory which I have tried to explain 

 is a tentative one, and further investiga- 



