314 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



hills. Of the rocks of Persia and Turkestan we know but little, so 

 the arc below this is dotted and slopes up to that beneath the Pamirs, 

 or central Asia. Here again our knowledge is far from precise, so 

 that the arc is dotted and is placed at the level of that of the average 

 of Asia, though it should probably be somewhat higher. Analyses 

 of Chinese rocks are few, but they would seem to be in general like 

 those of the Pamirs, though a trifle heavier. The Japan arc is above 

 that of China, and east of this we reach the wide arc beneath the 

 Pacific Ocean — the lowest of all, just as its rocks are the heaviest. 



After the journey around the world that we have just made in the 

 Northern Hemisphere it seems quite needless to describe that in the 

 Southern. (Fig. 5.) The reader may follow the correspondence 



Lot I0 -20°S 



Andes 



South 



America 



SamOClf:::}:-}:::: 



180 

 NewZealandi 



New Caledonia 



Queensland 

 Australio 



Brazil 

 Sao Paulo 



SI. Pauls Rocks 



^ Atlantic 

 Ocean 



jfe" St. Helena 

 West Africa 



Africa 



'Rift I/alley 

 East Africa 

 Madagascar 



Fig. 5. 



Indian Ocean 

 -Surface relief and specific volume. 



for himself, remembering that the available data along this zone are 

 far less numerous than along the northern; though we have good 

 knowledge of the rocks of Madagascar, 39 Eastern Australia and New 

 Zealand, and fair knowledge of those of the Andes and western 

 Africa and the Ethiopian Eift Valley. 



ROCK DENSITIES AND ISOSTAST. 



We come now to the final section of our study of the earth's crust, 

 the application of the data just presented to an important theory 

 regarding the stability conditions of the crust, the theory of isostasy. 



39 The average density of Madagascar is unquestionably less than that here given, as Is 

 shown by the many more analyses now available. The density 2.83 represents more prop- 

 erly that of Reunion, and thus that of the floor of the Indian Ocean. 



