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



" dip against tlie metamorphio foliated scliists at a high angle, 

 clearly indicating a fault." Now to the case of these strips of 

 younger rocks in Anglesea, such evidence seems more than inadmis- 

 sible, for the deposits are described as variable, coarse, and beach-like, 

 full of the debris of the adjoining rocks — facts, highly suggestive of 

 their having been formed against cliffs of these rocks ; but all this 

 is unnoticed in favour of a ' plane of marine denudation ' and 

 faulting ; no allusions ever being made to the information, or no in- 

 formation, derivable from the examination of the contact, or to the 

 possibility of any other mode of interpretation. Even among the 

 older rocks, the original features of the strata, as described by Pro- 

 fessor Eamsay, are similarly suggestive : within so small an area, 

 there are prodigious changes of thickness and of composition in 

 almost every group, facts suggestive of great inequalities of level 

 almost to the extent of partially detached basins of deposition; all 

 which circumstances might greatly modify the view put forward of 

 the section, and radically damage the presumptive and very indirect 

 evidence given for some of the great faults. All these features may 

 exist as described, and the high reputation of the author insures their 

 acceptance in full faith. I would only remark that the impartial 

 reader is left unconvinced, because of the unsatisfactory nature of 

 much of the evidence given, and the omission of evidence that 

 cannot be dispensed with. ^ 



If such great lines of movement are not fissures and loci (at least 

 potentially) of friction, they are not faults, according to the accepted 

 definition, and some term should be adopted to distinguish them 

 from simple faults. Some such apparent cases that I know of in 

 India are, I believe, nothing more than very steep limits of original 

 deposition, against which the younger deposits very rapidly over- 

 lapped older groups of the same series : subsequent settlement of 

 the deposits, with some lateral crushing, and accompanied by more 

 or less of very local faulting, has completed the similitude of a great 

 'boundary fault.' The distinction, however, makes an enormous 

 difference, for the simple announcement of a fault implies the former 

 undiminished extension of the deposits. To represent such a boun- 

 dary as a fault, on the strength of such small slips as may occur, 

 brings forward a subordinate, secondary, and trivial circumstance, 

 putting out of sight a most important, original, though now modified 

 relation of the adjacent rock masses. So long as faults are used — 

 though not avowedly or consciously, as a key to a Chinese puzzle 



^ Since writing as above, I have recollected and referred to a high authority upon 

 the very features under notice. Professor Haughton in a paper in the Proceedings of 

 the Geological Society of Dublin, vol. vi., 1854, describes the great Bangor and 

 Carnarvon ' fault,' in a manner reconcilable with the view I have suggested, he says 

 (p. 6) " The newer paleozoic beds dip towards the porphyry band, and appear to 

 have been originally deposited quietly upon its western slope. Some cause or com- 

 bination of causes — either alterations of relative level, or lateral pressure, or a com- 

 bination of both — gave to these beds a slight dip towards the south-eastern side, 

 and a line of strike making an angle of 20° with the porphyry band." I must note 

 my surprise that Professor Haughton, although in his paper deprecating " vague- 

 ness and looseness of expression," seems to use the word fault, out of its proper 

 geological sense, to mean a vein (p. 5.). 



