446 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



an explanation of the probable fact that the New England coast repre- 

 sents a more easterly part of the Paleozoic continent than is emerged 

 in the south. Why overthrnsting was carried to a greater extreme in 

 eastern New York than in, say, Virginia may be due to that ancient 

 peninsular projection of the Canadian shield — Adirondackia — against 

 which deeply rooted buttress the sedimentary rocks were piled. As ex- 

 plained on page 565, the Catskill mass of Devonian rocks probably also 

 cause(i extraordinary overthrusting of Eopaleozoic deposits to the east 

 of it. 



Relation of lateral creep of continental platform to suboceanic spread- 

 ing. — The theory of lateral spreading of the continental platforms sug- 

 gested by Chamberlin and Salisbury*^ is based on the apparently 

 reasonable supposition "that during profound body deformations the 

 continental segments may be pressed up, in portions at least, beyond the 

 plane of equilibrium, and that during the period of quiescence that fol- 

 lows they tend to settle back to the equilibrium plane." In discussing 

 the proposition the authors state "that continental spreading or creeping 

 will, within the limits of its operation, diminish the capacity of the ocean 

 basins, and so tend to cause the waters to overflow the lands." Further, 

 they say "such a movement would be unsymmetrically distributed, and 

 the result would be manifested in slow, quiet warping." 



Confined to the surface of the continents within the marginal areas 

 which have been more especially affected by "suboceanic spreading" 

 (see page 435), the process of continental creep may well have been an 

 important factor in the production of the gentle warpings and differ- 

 ential tiltings of the median areas of frequent uplift described on pre- 

 ceding pages. As stated by Chamberlin and Salisbury, the force of 

 gravity back of the tendency to "creep" is "perpetually in readiness to 

 act." At the same time the forces causing suboceanic spreading are 

 no less continuous in their operation. The two processes then are forever 

 at war, the issue wavering between them. The latter at times forces the 

 whole continent out of the water, the former ever tends to depress the 

 median portions. Either movement would naturally result in gentle 

 warping and buckling of the median areas, and the alternating dominance 

 of one and then the other probably would produce results sufficiently 



behind the newer basal sheet, but that they yielded to an inherent gravitational tendency 

 to backsliding. Warping, normal faulting, and locally even considerable folding and 

 slight underthrusting should therefore be expected in the sedimentary rocks that were 

 laid down on marginal areas after the belt of folding had migrated inland beyond their 

 location. Retreat movement of this kind is suggested by the folded Pennsylvanian de- 

 posits in the Narragansett basin. 



*« Geology, vol. ii, 1907, pp. 131, 233-236. 



