H. B. Medlicott — On Faults in Strata. 343 



ently justify the demand for the production of this direct evidence 

 in the case of asserted boundary -faults, the indirect evidence being 

 in these cases always more or less inconclusive. 



The indirect evidence for faults is the transverse separation of 

 rock-masses that were originally continuous or contiguous. For 

 those who can accept the proofs of such former conditions, this 

 evidence is perfectly demonstrative ; and it gives the only means of 

 calculating the amount of the throw. It may be said that the fault 

 is, after all, wholJy dependent upon that prior and distinct ques- 

 tion — whether the rocks in contact were, or were not, originally so 

 {i.e. from the time of formation of the younger) ; for, as has been 

 observed, the direct evidence may give no certain answer upon the 

 amount of the throw, which mdiy be so slight as to be unworthy of 

 notice unless in some very special inquiry, where it would only rank 

 as a simple fissure. 



In determining the existence, position, and amount of an unseen 

 fault in stratified rocks, great attention has to be directed to the dips 

 and strike of the strata. I would thus partially account for an error 

 that seems to have gained credit with many field geologists, that 

 the existence of a fault can be detected from a one-sided examination 

 of the dijDS — from the arrangement of the strata on one side of the 

 supposed fault. The question is a vital one, for it obviously comes 

 most frequently into operation in important cases of apparently great 

 " master-faults," where later sedimentary rocks are in juxta-position 

 with crystalline, or widely distinct masses. The origin of such a 

 notion is, perhaps, traceable to the " fixed idea" of the fundamental 

 principle of Geology, — the sztper-position of the younger deposits. 

 However this may be, it would seem to be a rule with many ob- 

 servers at once to set down as a fault any abrupt, steeply inclined, 

 junction of rocks of different ages, and showing signs of disturbance. 

 A reference to any actual area of the earth's surface would, in the first 

 place, remind one that in any basin of deposition new strata must 

 often abut against steep surfaces of the old sujDporting rock. And 

 upon the second point (that of disturbance), a brief consideration 

 would satisfy one that a subsequent lateral compression, modified 

 by slightly varying conditions of resistance, might produce any 

 imaginable complexity of dips in the younger strata near such 

 contact. 



There is another point in connection with faults upon which our 

 preconceptions are allowed to interfere with true observation. From 

 the manner in which we sometimes see the faults of a district 

 forced into " systems," according to the direction they riui in, one 

 might suppose that the only chance of explaining them depended 

 upon this being done ; or, that the observer's work were accom- 

 plished, and a scientific result attained, by the production of any- 

 thing that can be called a system. These systems are, indeed, some- 

 times seen to be self-destructive where they manage, more or less, to 

 lose the compass between their limits, but the mere attempt is to a 

 great extent based upon a petitio princijni ; to cause such symmetr3' of 

 arrangement, it is assumed that in the production of faults, such wide- 



