3° 



NA TURE 



[Nov. 13, iJ 



to time during the summer, reports had been made to me 

 of the progress of the survey, but, though from the published 

 descriptions of that tract, I was aware that its structure 

 must be singularly complicated, and although apprised of 

 the conclusions to which the surveyors, step by step and 

 almost against their will, had been driven, I was hardly 

 prepared for the extraordinary geological structure which 

 the ground itself presented, or for the great change 

 necessitated in the interpretation of the sections as given 

 by Murchison. 



No one cursorily visiting the ground could form any 

 notion of its extraordinary complication, which could only 

 be satisfactorily unravelled by patient detailed mapping 

 such as had never yet been bestowed upon it. With every 

 desire to follow the interpretation of my late chief, I criticised 

 minutely each detail of the work upon the ground ; but I 

 found the evidence altogether overwhelming against the 

 upward succession which Murchison believed to exist in 

 Eriboll from the base of the Silurian strata into an 

 upper conformable series of schists and gneisses. The 

 nature of this evidence will be best understood from 

 the subjoined report, which, at my request, Messrs. 

 Peach and Home have prepared. As the question of the 

 succession of the rocks in the North- West Highlands is 

 still under discussion, I think it right to take the earliest 

 opportunity of making this public declaration. It would 

 require more space than can be given in these pages to 

 do justice to the views of those geologists, from Nicol 

 downwards, by whom Murchison's sections have been 

 criticised, and to show how far the conclusions to which 

 the Geological Survey has been led, have been antici- 

 pated. When the official memoirs are published, full 

 reference will be given to the work of previous ob- 

 servers, to which, therefore, no further allusion is made 

 at present. 



The most remarkable features in the Eriboll area 

 are the prodigious terrestrial displacements, to which 

 there is certainly no parallel in Britain. Beginning 

 with gentle foldings of the rocks, we trace these 

 becoming increasingly steeper on their western fronts, 

 until they are disrupted and the eastern limb is pushed 

 westwards. By a system of reversed faults, a group 

 of strata is made to cover a great breadth of ground 

 and actually to overlie higher members of the same series. 

 The most extraordinary dislocations, however, are those 

 to which for distinction we have given the name of 

 Thrust-planes. They are strictly reversed faults, but with 

 so low a hade that the rocks on their up-throw side have 

 been, as it were, pushed horizontally forward. The dis- 

 tance to which this horizontal displacement has reached 

 is almost incredible. In Durness, for example, the over- 

 lying schists have certainly been thrust westwards across 

 all the other rocks for at least ten miles. In fact, these 

 thrust-planes, but for the clear evidence of such sections 

 as those of Loch Eriboll, could not be distinguished from 

 ordinary stratification-planes, like which they have been 

 plicated, faulted, and denuded. Here and there, as a 

 result of denudation, a portion of one of them appears 

 capping a hill-top. One almost refuses to believe that 

 the little outlier on the summit does not lie normally on 

 the rocks below it, but on a nearly horizontal fault by 

 which it has been moved into its place. Masses of the 

 Archaean gneiss have thus been thrust up through the 

 younger rocks and pushed far over their edges. When a 

 geologist finds vertical beds of gneiss overlying gently 

 inclined sheets of fossiliferous quartzite, bhale, and lime- 

 stone, he may be excused if he begins to wonder 

 whether he himself is not really standing on his head. 



The general trend of all these foldings and ruptures is 

 from north-north-east to south-south-west, and the steep 

 westward fronts of the folds show that the terrestrial 

 movement came from east-south-east. Corroborative 

 evidence that this was the direction of the movement is 

 furnished by a series of remarkable internal rearrange- 



ments that have been superinduced upon the rocks. 

 Throughout the whole region, in almost every mass of 

 rock, altogether irrespective of its lithological characters 

 and its structure, striated planes may be noticed which 

 are approximately parallel with the thrust-planes, and are 

 covered with a fine parallel lineation, running in a west- 

 north-west and east-south-east direction. These surfaces 

 have evidently been produced by shearing. Again, many 

 of the rocks near the thrust-planes, and for a long way 

 above them, are marked by a peculiar streaked structure 

 which reminds one of the fluxion-lines of an eruptive rock. 

 The coarse pegmatites in the gneiss, for example, as they 

 come within the influence of the shearing, have had their 

 flesh-coloured feldspar and milky-quartz crushed and 

 drawn out into fine parallel lamina; till they assume the 

 aspect of a rhyolite in which fluxion-structure has been 

 exceptionally well developed. The gneiss itself coming 

 into the same powerful mill has acquired a new schistosity 

 parallel with the shearing-planes. Hornblende-rock has 

 been converted into hornblende-schist. Moreover, new 

 minerals have likewise made their appearance along the 

 new divisional planes, and in many cases their longer 

 axes are ranged in the same dominant direction from 

 east-south-east to west-north-west. 



Murchison believed that the Silurian quartzites and 

 limestones of Eriboll pass up under, and are conform- 

 ably overlain by, his upper gneiss. It is quite true that 

 they are so overlain ; but the overlying rocks, instead of 

 having been regularly deposited on them, have been 

 pushed over them. What, then, are these overlying rocks ? 

 Though they have undergone such intense alteration 

 during the process by which they were moved into their 

 present position that their original characters have been 

 in great measure effaced, lenticular bands occur in them 

 which can certainly be recognised. Some of these bands 

 are unquestionably parts of the Archaean gneiss ; others 

 are Silurian quartzite, and in one case we can detect a 

 large mass of the Upper Durness limestone. Traced 

 eastwards, however, the crystalline characters become 

 more and more pronounced until we cannot tell, at least 

 from examination in the field, what the rocks may origin- 

 ally have been. They are now fine flaggy micaceous 

 gneisses and mica-schists, which certainly could not have 

 been developed out of any such Archaean gneiss as is 

 now visible to the west. Whether they consist in part of 

 higher members of the Silurian series in a metamorphic 

 condition remains to be seen. The occurrence of a band 

 of crystalline limestone and calcareous schist, which has 

 been traced for many miles above the great thrust-plane, 

 certainly suggests that it represents the upper part of the 

 calcareous Durness series attenuated and altered by the 

 intense shearing which all the rocks have undergone. 

 This much at least is certain, that the schistose series 

 above the thrust-plane is partly made up of Silurian 

 strata, and has received its present dip and foliation since 

 Silurian time. 



Having satisfied myself that Murchison's explanation 

 of the order of sequence could not be established in 

 Eriboll, I was desirous to see again, in the new light 

 now obtained, some of the Ross-shire sections for the 

 description of which I am responsible. Had these sec- 

 tions been planned for the purpose of deception they 

 could not have been more skilfully devised. The paral- 

 lelism of dip and strike between the Silurian strata and 

 the overlying schists is so complete as to prove the most 

 intimate relationship between them ; and no one coming 

 first to this ground would suspect that what appears to be 

 a normal stratigraphical sequence is not really so Bui 

 the clear coast-sections of Eriboll, where every disloca- 

 tion is laid bare, have now taught me that I have been 

 mistaken, for the parallelism in question is not due to 

 conformable deposition. The same kind of evidence of 

 upthrust and metamorphism which these coast-sections 

 reveal can be traced southwards for a distance of more 



