116 W. T. Blanford—On Faults in Strata. 



M^dlicott's remarks. The subject is one of very great interest and 

 importance, indeed it may be said to be a fundamental question in 

 Geological Surveying ; and, as I differ in opinion from Mr. Medlicott, 

 I wish to offer a few remarks in defence of the faults. 



The first point upon which I should join issue with Mr. Medlicott 

 is his demand for direct evidence of friction surfaces or slicTcensides . 

 I have not myself noticed these, as a rule, in undoubted faults — I 

 mean faults exposed in cliff or mine-sections, in which the beds are 

 distinctly seen at different horizons at the opposite sides of the fault — 

 and I believe that my experience in this respect will be corroborated 

 by most field-surveyors. Mr. Medlicott refers to the parallel case of 

 glacial action, but to this I object entirely, for two reasons : — 



1st. The circumstances under which friction-surfaces produced by 

 glaciers become visible involve the partial or total disappearance of 

 the glacier — a process not necessarily attended by any important 

 weathering of the rock-surfaces against which the glacier impinged ; 

 but the removal by denudation of a large portion of the rock from 

 one side of a fault, without any weathering of the opposite surface, is 

 almost an impossibility. In ninety-nine eases out of a hundred, the 

 rocks on both sides of a fault weather evenly, or nearly so; the 

 harder rock rising from the line of fault gradually. In order to find 

 the parallel to this, the rock bounding a glacier should be removed 

 pari passu with the glacier itself, no bergschrund or fissure inter- 

 vening. Under these circumstances it would be rather a difficult 

 matter to find roches moutonnes, or grooved and polished surfaces. 



2nd. The polisbed and grooved surfaces, which prove the former 

 existence of glaciers, are invariably on the hardest and least easily 

 weathered rocks, and are, geologically, of recent origin. No one 

 supposes that any such evidence could now be found, even of 

 Pliocene glaciers; but faults are of all ages, the vast majority being 

 Pre-tertiary at least. Again, every field geologist, who has worked 

 in countries where granitic and other crystalline rocks occur, must 

 have noticed the remarkable manner in which smooth surfaces, freely 

 exposed, resist weathering. Blocks of granite may often be seen, the 

 upper exposed surfaces of which are fresh and unaltered, whilst the 

 sides and lower surfaces, which have been in contact with turf soil 

 or other masses of rock, are much disintegrated. The former, free 

 exposure, is the condition of the glacier- worn surfaces, most of which 

 are on crystalline rocks ; they have never been exposed to any more 

 destructive influence than that of pure water in the shape of rain : 

 the latter, that of contact with other rocks, is the condition of the 

 polished or grooved surfaces in a fault. Moreover, a fault is usually 

 more or less a plane of percolation for waters, often, if not invariably, 

 charged to a certain extent with gases and salts in solution, and thus 

 capable of effecting a chemical change in the rocks percolated, which 

 will certainly destroy friction-surfaces; and these destroying in- 

 fluences have, in the vast majority of faults, been at work, not since 

 the glacial epoch merely, but through the countless ages of geological 

 time which have elapsed since the Mezozoic or even the Palaeozoic 

 formations were consolidated. 



