160 GEIGER AND KEITH — STRUCTURE OF THE BLUE RIDGE. 



and dip in adjacent beds and wide differences in adjacent sections are equally 

 good proof. Both have been described by Mr. Hayes, and both have been 

 observed by the junior author. 



What is the case here? Fossils occur in the Shenandoah limestones show- 

 ing their equivalence to the Chazy-Calciferous of New York. Outside of the 

 limestone none have been found ; therefore we must look for unconformity 

 to prove that appearances are not true, and we must find it between the 

 limestone, the datum formation, and the sandstone, which is the top of the 

 section. 



Actual Conformity. — Of unconformity, however, in that place there is not 

 the least suggestion. Between the shale and the granite-schist series east of 

 the Blue ridge there is unconformity of the ordinary type of deposition, but 

 that does not affect the question, (a) In individual sections the limestone is 

 interbedded with shale, the sandstone with the shale and the shale with the 

 sandstone. (6) It is, moreover, the same horizon of the limestone in contact 

 with the shale. Most strata of the limestone, it is true, cannot be positively 

 identified, but a thin bed of white marble and slate can be, and that occurs 

 repeatedly only 400 feet below the shale, (c) The general section is a unit 

 from one end of the region to the other; limestone, 500-600 feet of shale, 

 sandstone. Fifteen different sandstone areas next the limestone along thirty- 

 five miles give the same sequence. This thickness of shale is small, but it is 

 very uniform, and by its very smallness makes the uniformity more certain, 

 because it is more accurately measured and would be more easily removed 

 by a fault. 



Limitations of possible Faulting. — The possibility of a fault is practically 

 removed, even where there is only a single sequence of limestone and shale, 

 (a) Where, as in the sections shown in plate 5, there are four or five repeti- 

 tious in a direct cross-section of as many miles, the case is so extreme that 

 the hypothesis of a fault is entirely excluded. In order to have existed 

 without impairing the symmetry of the folds the fault must have followed 

 the course of the shale bed 500 feet thick, (b) The present width of this 

 possible fault plane would be three and a half miles at the end of the Blue 

 ridge, with the corrugated surface of the shale bed. Its actual width, meas- 

 ured along the shale bed, would be five miles. This is entirely within the 

 reach of the great overthrust faults known in the Appalachians, so far as 

 mere distance of thrust is concerned, but the character of thrust is entirely 

 different, (c) Various broad thrust planes in Tennessee studied by Hayes 

 and the junior author show a marked unconformity in stratigraphy along 

 the line of break. While the difference of dip rarely is much, the rocks of 

 the down-thrust lie along the bevelled edges of the up-thrust and vice versa. 

 Sometimes one, sometimes the other is bevelled, but the over and under series 

 bear no relation to one another. Here the case is quite different. Instead 



