Davis.] 120 [November 1, 



pie tilting must explain the entire disturbance. Hitchcock took 

 this view ; and he is followed by Leconte. 1 The difficulties in the 

 way of accepting the explanation are again the enormous amount of 

 erosion it involves ; the enormous thickness of sandstones it requires; 

 but more important still, the incorrectness of the supposition that 

 faults and folds are absent. Both faults or folds can either be 

 proved or shown to be of so likely occurrence, that there is no 

 necessity of supposing the unbroken monoclinal : a faulted and 

 gently folded monoclinal. corresponds better with the observed 

 facts and avoids the difficulties of great thickness and great erosion 

 of strata. It is worth noting that Rogers, Hitchcock, Kerr and 

 Russell were rather driven to their theories in order to avoid diffi- 

 culties, than called to them by direct evidence ; and theories based 

 thus on absence of any other means of explanation are not as a 

 rule well founded. The theory of the glacial erosion of lake- 

 basins and fjords is a case in point. 



How then are the faults and folds proved? 



Faults are occasionally determined by the direct visibility of 

 their fissure and displacement : and a few such of small throw 

 are known in the Triassic strips. Yet by far the greater number 

 of faults marked on geological maps and diagrams have never 

 been seen directly, but are only inferred to exist on account of 

 the displacement or reappearance of certain identifiable strata on 

 the two sides of their fissure. Such inferences may be well or 

 poorly based, according as the repeated stratum or series of 

 strata is surely or doubtfully identified at its two outcrops. 

 In attempting to apply this method to the study of the 

 Triassic belts, we are seriously embarrassed by the monotonous 

 character of the sandstones and shales that make up these depos- 

 its, so that the second outcrop of any given stratum cannot be 

 ascertained. Only in the Richmond coalfield has this difficulty 

 been overcome ; there the coal-beds have served as indices to the 

 displacements of the mass. Elsewhere, no faults of any consider- 

 able throw have hitherto been safely determined. 



The trap sheets here come in to play an important part. 



As stated at the beginning of the paper, it is generally taught 

 that these sheets were intruded between the sandstone layers ; 

 but this is in large part an error. While some of the sheets, 



i Elements of Geology, New York, 1878, 440, 441. 



