,S52 A. KEITH (3UTLIXES OF APPALACHIAN STRUCTURE 



deposits lies west of the main folded belt, although the latter is com- 

 monly nnderstood as the position of the main geosyncline. There is 

 considerable complexity, however, in the position of the geosyncline and 

 it was not the same at all j^eriods. In Algonkian time it lay to the east 

 of the present main uplift of the Appalachians ; in Lower Cambrian time 

 it lay slightly farther west, about the middle of the folded belt. In the 

 Upper Cambrian and Ordovician it was still farther west, but it was 

 still in the folded belt. The Silurian, Devonian, and Mississippian geo- 

 synclines had about the same jDosition, as stated above, mainly outside of 

 the strongly folded belt. 



Thus, the jDosition of the geosyncline progressed westward from Algon- 

 kian to late Carboniferous time. After the Appalachian revolution the 

 reverse is true, the Triassic geosyncline lying, for the most part, east of 

 the strongest Appalachian deformation, while the Cretaceous and Ter- 

 tiary geosynclines were entirely outside of it to the east. Where the 

 thickest deposits of the various geosynclines overlapjDed and piled up on 

 each other, or, in other words, about where the great Appalachian Valley 

 joins the Appalachian Mountains, there was presented the greatest op- 

 portunity for deformation, both in the amount of the sediments and in 

 the position of the rigid Precambrian floor. It is hardly necessary to 

 say that in that position is found the greatest deformation, including the 

 principal anticline of the Appalachians, the great border thrust faults, 

 and the metamorphism of the rocks. 



In Algonkian and early Cambrian time the sea lay to the east and 

 land to the west. This was reversed in the Upper Cambrian and was 

 continued through the Lower Paleozoic. In the Upper Cambrian, there- 

 fore, there began the rise of land on the east, which was the real beginning* 

 of the movements culminating in the Appalachian revolution. A similar 

 but reverse movement took place in the Mesozoic and Tertiary times and 

 appears to be still going on. These major movements — that is, the uplift 

 of the Lower Cambrian geosyncline and the depression of the Lower 

 Cambrian land on the west — have already been mentioned as directly 

 the opposite of the results which would follow from isostasy if that were 

 the only cause. The same is true of the great reverse movement of the 

 Mesozoic and later times which submerged the old land of Appalachia 

 and raised the geosynclines, which had been receiving sediment during 

 most of the Paleozoic. 



To sum up, the geosynclines and geanticlines were begun, were in- 

 creased, and then were reversed in their relative positions, not once but 

 again and again. Thus we are led to nearly the same conclusion for 



