THE STRENGTH OF THE EARTH'S CRUST 35 



GEOLOGIC SUGGESTIONS AS TO MAGNITUDES OF CRUSTAL STRESSES 



Submarine geanticlines and geosynclines: — Here will be con- 

 sidered some geologic illustrations of departures from isostasy, 

 arranged in order of harmonic wave-length. They are to be com- 

 pared with the results obtained from the study of the geodetic 

 data. Most of the geologic evidence is merely suggestive, not 

 conclusive, since diametrically opposite opinions are held as to the 

 probability of the visible load being offset by an invisible com- 

 pensation. As suggestions, however, they are none the less valuable, 

 and point the way to needed geodetic observations. 



The mountain folds advance from Asia over the floor of the 

 Pacific Ocean, forming the system named by Suess the Oceanides. 

 Mostly hidden beneath the ocean surface, they have been but little 

 affected by erosion. Their ridges and deeps mark the greatest 

 mountain reliefs of the globe. It is probable that here, if anywhere, 

 tangential pressures have forced the crust into folds whose height 

 combined with span is as great as the strength of the crust can 

 endure. To what degree the elevations and depressions are com- 

 pensated is, however, unknown, and the great arches are supported 

 in part by the lateral pressure of the ocean water. It is quite pos- 

 sible if not probable that appreciable changes of deep-seated 

 density may accompany the growth of such ridges, especially as 

 they mostly exhibit a volcanic activity and are to a greater or less 

 extent structures built up by igneous extrusion. It is not at all 

 probable, however, that they are completely or possibly even 

 largely compensated, but where the mountain folds and trough-like 

 deeps broaden into plateaus or anti-plateaus the presumption is 

 strengthened that the forms may there be isostatically compen- 

 sated to a large degree. Such plateaus or anti-plateaus cannot 

 then be used in the present argument. The ridges and troughs, 

 however, show in their forms, as has been stated, modes of con- 

 struction which are not conditioned on isostasy. Let attention 

 be turned then to the folds of the ocean floor. 



Passing from west to east, first may be noted between the 

 Philippines, Borneo, and New Guinea a complex of ridges and 

 basin-like deeps. The larger wave-length of that region runs from 

 300 to 500 km. The Ladrone Islands and Nero Deep give a 



