ROCKS OF THE NORTHERN HIGHLANDS. 357 



I endeavoured to show that the sections at Durness and in Assynt 

 did not display a true ascending series, the limestone in both loca- 

 lities being separated by a fault from the rock which was sup- 

 posed to conformably succeed it. On the relation of the limestone 

 to the quartzite I formed no opinion ; but in the two sections de- 

 scribed I found them brought together by faults. As the quartzite 

 passed with apparent conformity below the eastern gneiss on the 

 east of Loch Erriboll, I hinted, in accordance with the views of 

 Murchison, which I had not then seen reason to abandon, that the 

 quartzite might belong to the metamorphic series. Erom these 

 limited observations I carefully abstained from drawing a general 

 conclusion ; and I was convinced that no satisfactory result could 

 be achieved without a detailed survey. Accordingly I devoted a 

 fortnight in 1881, and two months in 1882, to the districts which 

 appeared most promising ; and I have now the honour to submit the 

 conclusions to which I have been led. 



The published theories on the relation between the Durness lime- 

 stone and the eastern gneiss are the following : — 



1. That the eastern gneiss conformably overlies the limestone, 

 and is of " Lower Silurian " age (Murchison, Geikie, and most 

 authors). 



2. That the eastern gneiss is merely the Hebridean brought up 

 east of the limestone along a line of dislocation and inversion 

 (JNicol). 



3. That the identity of the limestone of Assynt and Erriboll with 

 that of Durness has not been demonstrated, and therefore that the 

 eastern gneiss, though it conformably overlies the former, has not 

 been proved to be of " Lower Silurian " age (Heddle). 



The view which I have here to submit differs from all of the 

 above, but approximates. most nearly to that of Nicol. I maintain, 

 with that author, that the junction of the limestone with the eastern 

 gneiss is a line of faulting and inversion ; but I shall attempt to 

 prove that this gneiss is a distinct series, newer than, and resting 

 unconformably on, the Hebridean, that Nicol's "igneous rock" 

 overlying the limestone is usually a true gneiss, and that both the 

 older and younger gneissic systems have been brought up over the 

 limestone by great earth-movements. The eastern gneiss I propose 

 provisionally to name the " Caledonian." 



I. The Formations concerned in the Kesttli. 



The groups whose relations are to be discussed are the Hebridean 

 or Lewisian, the Caledonian, and the quartzo-dolomitic group which, 

 in view of the doubt which has been cast upon its age by the re- 

 searches of Prof. Heddle, I propose to call the Assynt series. 



a. Hebridean. 



In addition to the masses usually designated by this name, I in- 

 clude under it a large proportion of the " igneous rocks " of authors. 

 Recently the latter have been described by Dr. Heddle as a new 



Q.J.G.S. No. 155. 2d 



