DISTRIBUTIOX AXD ARRAXGEMEXT OF PRESEXT MOUXTAIXS '383 



an infliience exerted by the associated basins of tlie Indian, Southern, and 

 Atlantic oceans. 



The nuclei of these great regions are wide plateaus of low vertical 

 relief, with the exception of a few localities to be mentioned later. The 

 main examples are the "Canadian Shield/' the "Siberian Xucleus'" or 

 vertex of Suess, the "Scandinavian Nucleus," the Brazilian Plateau, the 

 Southwestern Australian Plateau, India, and a portion of Africa. These 

 regions are not broken b}' great gaps of tectonic origin arranged along 

 radii spreading or diverging from the older portion of the nucleus. 



The modifications suggested above are due to the existence of the high 

 Atlantic margin of the Scandinavian Plateau, the relatively high plateau 

 fringe to the "Canadian Shield'' within Labrador, and the ghats of 

 India. With these, perhaps, may be included the relatively high plateau 

 blocks in the northern portion of the West Australian Plateau.^ 



All these great continental shields are low in general altitude and are 

 encircled by broad land troughs filled with waste from the earth crests, 

 or undulations, forming these continental shields or nuclei. Beyond the 

 mighty plains of these lowlands and alluviated areas, namely, the savan- 

 nas, prairies, and plains, sympathetic rings of plateaus form outer lines 

 of land or earth crests. These are narrower and higher than the low- 

 lying nuclei, and they are broken to a considerable degree by warpings 

 and depressions which tend to be arranged along directions diverging 

 radially from the nuclei. Outer arcs occur beyond these in turn, and 

 they are narrower than those of the middle group ; they are of much 

 greater absolute height above the ocean bases or deeps; and they have 

 suffered great transverse warpings and faultings arranged along lines 

 extending radially from continental nuclei. This latter statement applies 

 to the western portion of the Pacific rather than to that of the eastern. 

 Examples of these transverse warpings are the separation of Xew Zea- 

 land, New Caledonia, Tonga, and Fiji. 



A few additional notes may be suj^plied concerning the arrangement 

 of the Asiatic plateaus, this continent being a t3^pe of the great land 

 blocks of the world. 



The Siberian nucleus is a low plateau diversified locally by a few high 

 ridges. The mountain ranges of the continent are arranged in arcs whose 

 convexities are directed mainly from the nucleus toward the Pacific and 

 Indian oceans.^ 



* Verbal communication from Sir Edgeworth David from information supplied to him 

 by A. Gibb Maitland. 



^ F. Bursley Taylor : Bearing of the Tertiary mountain belt on the origin of the earth's 

 plan. BuU. Geol. Soc. Amer.. 1910. 



