The Scottish Naturalist. 29 



tation of my late chief, I criticised minutely each detail of the 

 work upon the ground ; but I found the evidence altogether over- 

 whelming against the upward succession which Murchison believed 

 to exist in Eriboll, from the base of the Silurian strata into an upper 

 comformable series of schists and gneisses. The nature of this 

 evidence will be best understood from the subjoined report (which 

 will be found in Nature) 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 dec- 

 laration. 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 Sur- 

 vey have been led have been anticipated. When the official me- 

 moirs are published, full reference will be given to the work of 

 previous observers, to which, therefore, no further allusion is made 

 at present. 



The most remarkable features in the Eriboll area are the pro- 

 digious 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 dis- 

 locations, 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 distance to 

 which this horizontal displacement has reached is almost incredible. 

 In Durness, for example, the overlying schists have certainly been 

 thrust backwards across all the other rocks for at least ten miles. 

 In fact, these thrust-planes, but for the clear evidence of such sec- 

 tions as those of Eriboll, could not be distinguished from ordinary 

 stratification-planes, like which they have been plicated, faulted, 

 and denuded. Here and there, as the result of denudation, a por- 

 tion of one of them appears capping a hill-top. One almost 

 refuses to believe that the little out-lier on the summit does not 

 lie normally on the rocks below it, but on a nearly horizontal fault 



