474 H. B. MedlicoU — On Faults in Strata. 



as to resemble a mirror ; and for the most part the space between 

 the sides consists of coal, stone, and clay, ground to powder." That 

 great faulting can take place without scoring it is difficult to imagine. 

 It was only in support of this point that I alluded to glacial scratch- 

 ings ; being quite aware of the wide differences between the two cases. 

 Thus Mr. Blanford's first objection to the comparison is rather 

 irrelevant to any point at issue, as it only accounts in the usual way 

 for the scarceness of good contact-sections — a fact of which I have 

 had most wearying experience. The second objection, showing why 

 slickensides should be very generally destroyed in faults, as com- 

 pared to glacial friction surfaces, is slightly overdrawn : some of the 

 best glacial scorings I have ever seen were on ordinary limestone ^ 

 that had been exposed to the various modes of surface or sub-surface 

 weathering since the Glacial epoch. Nor is it, I think, a general 

 opinion that the action of deep underground percolation is more 

 destructive of form than atmospheric agents ; it is on the contrary 

 often eminently preservative, by substituting hard pseudomorphs of 

 what may have been originally in a very perishable condition. 



There is, however, no doubt that in many cases very strong direct 

 mechanical evidence of faulting has been preserved, enough (inde- 

 pendently of the contrary evidence that may be thus elicited) to 

 require some notice of the contact in every case of an asserted 

 boundary fault. 



It is not, I think, implied in my paper that this direct evidence is 

 the only, or an indispensable, proof for faulting : it is even shown 

 that this evidence may be most deceptive as to the amount of throw. 

 I thought it unnecessary to state that my paper had reference only 

 to the special point of contact-phenomena. There are several other 

 considerations connected with faulting to which I did not allude ; 

 among them, that of straightness, upon which Mr. Blanford lays so 

 much stress. That I fully recognize the importance of this character 

 is sufficiently shown by the fact that I have mapped and described 

 conditionally as great faults, and chiefly on this evidence of straight- 

 ness, several features in the Sub-Himalayan rocks (Mems. Geol. Survey 

 India, vol. iii., p. 134, etc.). I should still prefer to reserve the word 

 direct for the character of the contact : it is incomparably the sim- 

 plest, the most self-evident, and the least ambiguous; the appearance 

 described in the passage I have quoted could only be produced by 

 faulting. There is no other kind of evidence of which such an 

 exclusive statement could be made ; no one would affirm that a 

 sharply curved or a zig-zag fault is impossible — it is a question of 

 form, in which many conditions are involved. On the other hand, I 

 believe that Mr. Blanford has overstated the case against the occur- 

 rence of straightness in orographical features ; and that, therefore, 

 this character of straightness, as well as that of dips, is liable to be 

 simulated in natural boundaries. As an instance, I may quote from 



^ Hard Carboniferous Limestone surfaces may frequently be seen in the North of 

 England and in Scotland ; beautifully glaciated, but the glaciated surface has usually 

 been protected by a covering of drift or vegetable soil. We should doubt their pre- 

 Bervation on soft rock if exposed for long periods to the weather. — Edit. Geol. Mag. 



