20NES OF INTRUSION 411 



trusives were somewhat eroded and then sank under the Cretaceous sea 

 till more than 20,000 feet of limestone had accumulated. This thickness 

 of organic sediment marks a Caribbean deep and the flow of a warm life- 

 bearing ocean current across it. The subsidence to so great a depth after 

 considerable erosion is peculiar. No well defined positive element is 

 known to have been equally depressed since the late pre-Cambrian, at 

 least. 



Fixing attention for the moment on the Isthmian region, we may 

 recognize that it is with reference to the Caribbean and Pacific deeps 

 merely a narrow ridge, comparable in position with the Windward islands 

 or the ridges between the several deeps of the West Indian archipelago, 

 including those which are submerged. These are but the rims of great 

 deeps, and if the latter be considered as corresponding to negative ele- 

 ments of excessive density, there is scarcely space between them for a 

 notable mass of deficient density equivalent to continental lightness. 

 Nevertheless any two adjacent deeps are plainly not one, but are separate 

 individuals lying in a mass whose density, though apparently less than 

 theirs, is probably greater than that of the positive continental elements. 



If we may assume that each of the heaviest elements adjusts itself 

 individually to the stresses to which it may be subjected (whether they 

 be directly those of gravitation or the indirect effects of that force ex- 

 pressed in tangential stress), then the zone between two such elements is 

 liable to be one of excessive pressure, rising temperature, yielding, eleva- 

 tion, relief, and melting ; consequently a zone of intrusive and extrusive - 

 igneous rocks, as well as one of highly metamorphosed sediments. Such 

 a zone, it seems to me, is that of the Windward islands, that of the Isth- 

 mus at least as far north as Guatemala, and also that of the Aleutian 

 islands, Alaska. In the absence of any well defined positive element, 

 characterized by great unconformities and defined by marginal folded 

 sediments, I am inclined to regard most of Mexico as of the same type — 

 a region whose isostatic tendencies were negative, as compared with the 

 positive element of Arizona, but which has lost its isostatic balance and 

 been overelevated in consequence of displacement by the denser bodies 

 adjacent to it. 



Summary 



This article contains : 



(a) A statement of hypothesis according to which a continent is com- 

 posed of lighter and heavier bodies, whose relative vertical movements 

 have to some extent been in obedience to isostatic adjustment. The dif- 

 ferences of density are postulated as original differences, not accounted 



