410 B. AVILLIS A THEORY OF CONTINENTAL STRUCTURE 



a relation to displacements of the united western continent with refer- 

 ence to the Pacific, and, further, that the mechanical structures of the 

 subterrane have been effective in directing the movement of molten 

 masses, even to great distances from the margin of the oceanic basin. 

 Pre-Mesozoic conditions of intrusion, when better known, may shed more 

 light on the relations of particular zones to continental elements. To a 

 great extent the}^ still evade anal3^sis, but not entirely. 



The general structure of Alaska has been briefly described as consist- 

 ing of a narrow positive element, the Yukon plateau, and two zones of 

 folding, the one on the margin of the North Pacific, the other on the 

 edge of the Arctic negative element. The southern is a region of intense 

 folding, the northern a belt of moderate compression, as might be ex- 

 pected from the relative magnitudes of the two elements. Since an early 

 Paleozoic date, at least, a similar relation has existed between intrusive 

 activities on the southern and northern sides of the peninsula. The dis- 

 tribution in time and place can be made out from the correlation table 

 and general comments by Brooks.* It appears that the zone between the 

 rising Yukon element and the sinking Pacific basin was in Devonian, 

 Carboniferous, and Mesozoic times the scene of pronounced intrusion and 

 extrusion. 



Attention is called particularly to the Mesozoic intrusion of the great 

 granodiorite bathylith which extends from Alaska to southern British 

 Columbia and which ranks among the stupendous events of Cordilleran 

 history. The relation of the mass to the ancient elements of the conti- 

 nent and to their marginal zone after they became united is apparent. 



I conclude that the older Cordilleran intrusions in some degree exhibit 

 relations to the separate positive and negative elements, but that the late 

 Mesozoic and Tertiary eruptions are phenomena which can be explained 

 on this theory only by contrasting the united mass of the western North 

 America with the suboceanic mass beneath the Pacific. 



The Mexican-Isthmian extension of the continent may possibly be 

 homologous with Alaska, may consist of a positive niicleus, which is com- 

 pressed between the Pacific and Gulf negative elements. If so, the old 

 land area extended from Guatemala through Oaxaca and along the Pa- 

 cific coast toward Durango; but I have already referred to the descrip- 

 tions by Ordonez and Sapper, which lead rather to the conclusion that 

 the pre-Cretaceous rocks are chiefly metamorphosed Paleozoics and in- 

 trusives. The great unconformity of the region, so far as we yet know, 

 is that at the base of the Cretaceous. The metamorphosed rocks and in- 



* A. H. Brooks : Geography and geology of Alaska. Professional paper no. 45, U. S. 

 Geol. Survey, 1906, pp. 206 and 250. 



