394 B. "WILLIS A THEORY OF CONTINENTAL STRUCTURE 



Throughout this vast area the Laurentian gneisses or tlie sediments 

 into which they were intruded constitute the larger part of the surface. 

 It was submerged probably beneath the general Camljro-Ordovician 

 transgression and certainly to a great extent beneath the Siluro-Devonian 

 seas which spread over Arctic lands (Siberia, northern Europe, and North 

 America), while elsewhere in the globe there were notable phenomena of 

 emergence ; but these submergences and that of the Cretaceous were rela- 

 tively brief and scarcely affect the grand total of imconformities, which 

 ranges from Laurentian to the present. The net sum of relative depres- 

 sions and elevations is positive by a large amount. 



In the eastern part of the continent is the area of ancient schists and in- 

 trusives, whose relations have in the last few years been worked out chiefly 

 by Keith.* They extend from the Pennsylvania line southward in a 

 widening area, between the folded Paleozoics on the west and the metamor- 

 phosed Paleozoics on the east. Their surface sinks beneath the Mesozoic 

 and Tertiary of the Atlantic coastal plain. The role of this area in Pale- 

 ozoic history is well understood, in its broad outlines at least. From the 

 beginning of the Ocoee (pre-Cambrian) on through the Cambrian it was 

 an area of pronounced denudation. The Cambro-Ordovician sea prob- 

 ably swept over it entirely; but from late Ordovician time it stood for a 

 long period relatively high with reference to the Paleozoic mediterranean 

 on the west. At the close of the Paleozoic it became the scene of decided 

 mountain growth, and throughout the gucceeding eras it has either been 

 stable or has been affected by a positive movement. The sum total of 

 unconformities for this, which we may continue to call Appalachia, is 

 much the same as for Laurentia, although the integers differ. 



One of the most interesting speculations of American geology relates 

 to the extent of Appalachia during the Paleozoic. On the basis of Pale- 

 ozoic sediments, it may be credited with a wide expanse to the southeast 

 over the area of the Blake plateau, but the suggestion is not capable of 

 proof. 



We may next turn to the Llano region of Texas. Here the pre- 

 Cambrian rocks appear in consequence of relatively very recent upwarp- 

 ing, surroimded by Cambro-Ordovician, Carboniferous, and Cretaceous 

 deposits. The three great transgressions which have affected the conti- 

 nent submerged the region. For other periods the record is one of non- 

 sedimentation or erosion. The sum of unconformity and sedimentation 

 appears to have a positive remainder, but the net elevation is by no means 

 so large as in the case of Laurentia and Appalachia. The extent of this 

 rather neutral element is indefinite. It is bounded by the gulf of Mexico 



♦Geological Atlas of the U. S., Roan Mountain and other folios. 



