SIXTH REPORT OF THE DIRECTOR I909 145 



up, with the result that on either side two principal isles, Isle Wis- 

 consin [Do] and Isle Ozark [D3], Isle Adirondack [E,] and Isle 

 Appalachia [E^] were formed. These isles are distinctly paired. 

 On either side between the Canadian shield and the first isle a de- 

 pression formed, the Lake Superior basin [D^] and the St Law- 

 rence basin [E^], and other depressions between the first and second 

 isles. 



In the Paleozoic eastern basin a broad low anticline, the Cin- 

 cinnati-Nashville parma [B^] arose,^ exactly in the axial line 

 of the depression and in continuation of this line of elevation 

 the northern part of the basin sank down into an axial basin, 

 the Michigan subbasin [Bo]. On either side of the parma, 

 between the latter and the Precambric arms, two basins, the 

 East Central [B3] and the Eastern Interior basin [B4] were 

 formed, in such an arrangement that they converge southward 

 and are exactly symmetrical to the axial line of the eastern 

 basin. 



Chart IV illustrates the effect of the sub-Atlantic pressure, 

 the cause of the Appalachian folding and overthrusting. This 

 stress crushed or crumbled this symmetric structure from the 

 southeast, its influence being felt in the whole eastern portion 

 of the area. Following Claypole's earlier estimates, Willis, 

 [Geol. Soc. Bui. 1907, 18:404] remarks that "it is a moderate 

 statement to say that during the Appalachian revolution that 

 portion of the continent southeast of the Cumberland Plateau 

 rim moved northwestward at least 50 miles." ^ 



On account of its oblique direction to the north-south axis 

 of the Paleozoic eastern basin, the stress reaches deepest into 

 the latter in the south,, where it has clearly turned the Nash- 

 ville portion of the Cincinnati-Nashville parma aside. It further 



^ Siiess has termed such broad warpings " parmas." 



' Dr Willis arrives at this estimate in the following way: It is well 

 established that the folding of the Paleozoic strata in the Appalachian 

 zone corresponds to a narrowing of the zone by 35 miles or more — that 

 is,, the Blue ridge approached the Cumberland plateau from a distance 

 of 100 miles to within 65 miles. The general effect may best be de- 

 scribed as a composite overthrust from southeast toward northwest. 

 Keith's recent investigations show that overthrusts of equal or greater 

 displacement traverse the gneisses of the Smoky mountains. Hence 

 it is a moderate statement to say that during the Appalachian revolu- 

 tion that portion of the continent southeast of the Cumberland Plateau 

 rim moved northwestward at least 50 miles. 



