342 H, B. Medlicott — On Faults in Strata. 



is quite misrepresenting the said features to describe tliem simply as 

 fault-boundaries. It would be comparatively useless, in an attempt 

 to ventilate this question, to illustrate it exclusively from sections in 

 India ; but it may, perhaps, be presumed that professional practice 

 in this country is not very far behind the age, and that it will be easy 

 to find illustrations within the reach of the centres of scientific 

 research. I cannot make a better selection for this purpose than the 

 highly authoritative work lately brought out by Professor A. 0. 

 Eamsay, on the Geology of North Wales. 



The definition of a fault is as cut and dry as could be desired ; a 

 verbal completeness that may, in this case, as in so many others, have 

 helped to the neglect of the thing itself. It is a fissure along which 

 relative displacement of the adjoining rock-masses has taken place. 

 Although considerable latitude must be allowed in the application of 

 the term, from the clear-cut fissure, with throw, to what is merely an 

 overstrained contortion, geologists have unanimously given up the 

 miner's purely technical use of the word, and have made transverse 

 dislocation the characteristic of a fault. I would limit the direct 

 evidence for such a phenomenon to the fissure itself, with what signs 

 it may have of the supposed mechanical action, such as friction- 

 surfaces. As regards 'fault-rock,' it would be difficult to define the 

 composition or structure of a rock that would necessarily imply 

 faulting : veins often become filled in the most heterogeneous manner 

 with the debris of their walls, and thus a breccia of any form may 

 result. It is conceded, too, that the evidence of friction-surfaces 

 may be slight, or even superficially deceptive : a very small move- 

 ment, that would hardly deserve notice on a geological map, 

 as a fault, might cause much scoring of the opposed surfaces : 

 or, it might often happen in a fault that the friction would be 

 localised at the projecting parts of the surfaces, having large patches 

 without any trace of such action. Still, this direct instance should 

 be sought for. When it is considered what safe and important 

 records of this kind have been left by glacial action, it is surely to be 

 expected that great section-surfaces of the earth's crust passing 

 against each other for several thousands of feet, under the forces 

 that must be engaged in the production of such amounts of motion, 

 should leave some traces proportionate to the action. But it is 

 precisely on these points that I would ask for information, to be 

 delivered from the uncertain, because misplaced, use of a priori 

 resources. It would be a great aid to the observer, if it were es- 

 tablished that the absence of any evidence of friction in a fissure, or 

 plane of contact of dissimilar rocks, gave presumptive proof 

 against much faulting. Should it on the other hand be proved from 

 independent evidence, that great faults have occurred without leaving 

 any direct traces, the fact will at least suggest important inferences 

 upon the nature of the phenomenon; such as, that the vertical 

 motion cannot have occurred simultaneously with great lateral 

 pressure. I will presently indicate the important evidence on the 

 other side that may be expected from the examination of the actual 

 contact of rock-masses in juxtaposition, and which would independ- 



