142 C. W. HAYES — OVERTHRUST FAULTS OF THE APPALACHIANS. 



to the steeper limb of the anticlinal, becomes common. Recent study in 

 the southern Appalachians has shown a modification of these well-recog-' 

 nized types, namely, broad overthrust faults which, as developed in north- 

 western Georgia, are comparable in magnitude with those of the Scottish 

 highlands and the Rocky Mountains as described by Geikie * and McCon- 

 nell.f 



Stratigraphy of the Region. 



The strata of the region sho\vn in the accompanying map (plate 2) em- 

 brace representatives of all the larger groups of the Paleozoic from the 

 Cambrian to the Carboniferous, inclusive. The formations appearing in the 

 stratigraphic column, figure 1, represent a purely lithologic classification, 

 and no attempt is made to correlate them with coordinate subdivisions in 

 other regions. 



Considered with reference to their structural relations, the rocks of the 

 region fall into several groups of strata having varying degrees of rigidity. 

 The ability of a given thickness of rock to transmit a lateral thrust without 

 folding — that is, the rigidity — depends chiefly on the absence of bedding 

 planes. The composition of the rock itself is an important but subordinate 

 factor. 



The upper portion of the column — including the Coal Measure sandstone, 

 the Bangor limestone and the Oxmoor sandstone — forms a group of strata 

 with many bedding planes on which motion may take place with compara- 

 tive ease. Individual beds of sandstone or limestone are frequently quite 

 massive, but these are separated at short intervals by beds of fissile shale. 

 Hence this complex mass of strata possesses a rigidity below the maximum. 



The next division consists of the uniformly thin-bedded Floyd shales. 

 Here the number of planes on which motion may take place is the greatest 

 possible, and consequently the rigidity of the mass is at a minimum. 



The division below the Floyd shales embraces the Fort Payne chert, the 

 Chattanooga black shale, the Rockwood formation and the upper part of the 

 Chickamauga limestone. As in the case of the first mentioned division, bed- 

 ding planes are abundant, yet the separate beds are composed chiefly of 

 sandstone and limestone; and the mass of strata therefore possesses an inter- 

 mediate rigidity. 



The next division of the column embraces the lower part of the Chicka- 

 mauga limestone and the Knox dolomite. In the latter, true bedding planes 

 are almost wholly wanting. The formation consists of from 3,500 to 4,500 

 feet of massive, cherty, dolomitic limestone. Together with the lower por- 



* " The Crystalline Rocks of the Scottish Highlands ; " A. Geikie, Nature, vol. XXXI, 1884, p. 29. 

 Also " Report on the Recent Work of the Geological Survey in the Northwest Highlands of Scot- 

 land;" A. Geikie, Quart. Jour. Geol. Soc, vol. XLIV, 1888, p. 378. 



t" Report on the Geological Structure of a Portion of the Rocky Mountains; " R. G. McConnell, 

 Geol. Surv. Canada, Annual Report for 1886, part U. 



