408 B. WILLIS A THEORY OF CONTINENTAL STRUCTURE 



tions of North America outside the Canadian protaxis have been pushed 

 northwestward, northward, and northeastward toward it, until the conti- 

 nent may be said to consist of tliat great nucleus with a group of smaller 

 bodies arranged about it. So with Asia, though in more striking man- 

 ner. Suess has distinguished the IJrscheital in central and northern 

 Siberia and compared the structure of the continent to an arrangement 

 of garlands- pendent from it.* A study of the continental history of 

 Asia according to our present information shows that the northern land- 

 mass gained in area by additions on the south and east through the north- 

 westward and northward movements of smaller positive elements, f. 



The general structure of Europe may be roughly expressed by the 

 statement that its southern and western elements have been pushed north- 

 ward and eastward toward the Eussian nucleus. 



If now we consider the distribution of land and water throughout the 

 globe, we find a southern oceanic hemisphere and a northern continental 

 hemisphere.! According to the evidences of isostatic equilibrium, this 

 arrangement corresponds to that of lighter and denser elements; and 

 according to the theory of suboceanic spread here entertained, the gen- 

 eral northward movement of continental elements and the effects of 

 tangential pressure exhibited by the ancient gneisses and schists of the 

 great northern nuclei are due to the stress exerted by the heavier sub- 

 oceanic mass of the water hemisphere. 



Considering the immense embayments of the northern Pacific and 

 Atlantic oceans among the continents, it is obvious that no symmetrical 

 relation of stresses can exist; yet, in view of the vast eras during which 

 the conditions have endured, there can be no doubt that the stresses of 

 which the Arctic is approximately the center have long since tended 

 toward an equilibrium. Hence the stability of northern lands. 



Zones of Intrusion 



In discussing tlie zones of intrusion and extrusion of North America, 

 the immediate purpose is to enumerate those which appear to have rela- 

 tions to the positive and negative elements that have been distinguished. 



It is well known that intrusives constitute a very large proportion of 

 the rocks of Laurentia, Appalachia, and similar areas. The fact is in- 

 terpreted by Van Hise§ and also by Chamberlin|| as evidence that the 



* Das AntiUtz der Erde, vol. 3, chap. 3. 



t BaUey WiUis : Research in China, vol. 2, Systematic Geology. Publication no. 54, 

 Carnegie Institution of Washington, 1907. 



t Stieler's Atlas, 1905, Karte 3, Land und Wasser Halblsugel. 



§ C. R. Van Hise : A treatise on metamorphism, p. 707. 



II T. C. Chamberlin and R. D. Salisbury : Manual of geology, 1st edition, vol. 2, pp. 

 130-131. 



