EVOLUTION OF THE EARTH'S OUTLINES. 735 



tinents, or comparatively stable areas, in relation to the oceans, or 

 more subsiding areas ; the second, structural, — the system of cleavage- 

 structure ; the third, dynamical, — the tension in the crust itself accu- 

 mulating most through the subsiding of the oceanic basins. 



The features of the oceanic basins and the position of the con- 

 tinents determine the general bearing of the tension ; and the 

 cleavage-structure, or courses of weakest cohesion, tends to give 

 direction to its effects. The North Atlantic follows one of the 

 cleavage-courses, the Pacific another (page 36) ; North America is 

 bounded by the two, and hence its triangular form. See, further, 

 on these lines, pages 30 to 39. 



The courses of the rents or uplifts in such a crust will depend 

 on the direction of the tension in connection with the cleavage ; 

 just as in a piece of cloth the rents from stretching it will vary 

 with the direction of the force. 



Force exerted at right angles to the lines of structure, and equal 

 along the line, would produce a straight series of rents or uplifts 

 (figs. 11, 12, p. 19). 



If not equal along a given line, the rents might together make 

 an oblique or curving series (figs. 14, 15, p. 19). If the tension were 

 oblique to the structure-courses, the series of rents would be oblique, 

 and, as above, either straight or curved. 



Hence curves are necessarily in the system. 



The coincidence between the trend of the Pacific (northwest and 

 southeast), the mean trend of the Pacific islands (p. 34), and the 

 axis of the coral-island subsidence (p. 587), shows that the ocean 

 in its movements has been one great area of oscillation. The cen- 

 tral curving range, 6000 miles long, lies on the southern side of the 

 axis of this great approximately elliptical area. The tension in this 

 subsiding area itself appears here to be the cause of the long curve. 



The double or triple system of curves around Australia, from 

 New Hebrides, or perhaps northern New Zealand, to New Guinea 

 and Timor, are such as might arise from the Pacific tension, acting 

 against that stable continental area of Australia ; for they are con- 

 centric with it ; and the branch of the central Pacific chain leading 

 off westward through the Carolines has been shown, on page 34, to 

 conform to this Australian system. The rising curve from Java, 

 through Sumatra, suggests that here the oceanic basin on the other 

 side of Australia (the Indian Ocean) has brought tension to bear 

 against the Asiatic continental area ; and this is further confirmed 

 by the fact that the deep-water channel separating the Australian 

 seas from the Asiatic passes just north of New Guinea and Timor 

 and south of Java. 



