H. B. Medlicott — On Faults in Strata. 475 



Mr. F. E. Mallet's description of the Vindhyan rocks (Mem. G-eol. 

 Survey India, vol. vii., p. 106), " The escarpment of the Kymon range, 

 which faces the same for more than 200 miles between Eotasgarh 

 and Bilheri, is nearly a straight line for the entire length without 

 any indentation except the curve at Bidjigarh." Other instances 

 might be given, though of less extent, from the Eajgir and Grhidaur 

 hills. The word straight is quite as strictly applicable here as to the 

 coal-measure boundaries ; but it is right to note that the latter are in 

 gneissose rocks. In steep ranges, formed along a steady strike, 

 transverse drainage-cuts are apt to be few and narrow : when to 

 such considerations we add from the other side the broken nature of 

 the observations upon which geological boundaries must often be 

 mapped, and that such interruptions and concealments would be most 

 likely to coincide with original indentations of the boundary, I cer- 

 tainly must consider it a safe and necessary principle, rather than a 

 false one, to require some reference to the evidence of the actual 

 contact. No one would question the fact of a fault if he believed 

 the feature to be in fact as straight as it appears when drawn with a 

 ruler on a geological map. But every field geologist knows how 

 rarely it is possible really to vouch for this, so that, unless corrobor- 

 ative evidence is given for a fault, it is difficult for any one who 

 understands the possibilities of error, to accept the feature as demon- 

 strated. I cannot but feel that on the principle here advocated my 

 colleague and I are really at one. 



In objecting to my example from the Garo Hills, Mr. Blanford 

 again slightly departs from the special object the case was used to 

 illustrate, viz., the widely different inferences to be drawn from the 

 partial and from the full observations of that single section ; without 

 the examination of the contact at that spot, the practice most in vogue 

 amongst geologists would, from the rest of the evidence, assume a 

 fault, and place it at the contact ; whereas the contact-section clearly 

 proves that there is no fault there, and as clearly suggests that the 

 great relative movement took place by flexure. No one would con- 

 sider the question of the whole boundary finally settled upon the 

 evidence of a single section ; nor would any one apply the evidence 

 of a clear contact-section to others, unless there were agreement of 

 the more general and easily observed features. It is, on the other 

 hand, admissible to give its full weight to all the evidence in hand ; 

 and it would be an unwarrantable restriction to • our pi-actical rules 

 not to admit one clear section as a clue in the interpretation of several 

 obscure ones of the same general feature. The evidence of straight- 

 ness, upon which Mr. Blanford would still introduce a master-fault 

 here, would be in such a case doubly liable to error, for flexures are 

 by their nature as liable to the law of straightness as faults are. 

 Professor H. D. Eogers, who paid so much attention to the details of 

 stratigraphy, went so far as to say that all faults are upon broken 

 flexures. And it must be noted that the natural boundary exposed 

 by the flexure in question is a floor, not an edge, of deposition ; and 

 may be a "plane of marine denudation," thus escaping from the veto 

 Mr. Blanford would place upon straight natural boundaries. A de- 



