The Scottish Naturalist. 31 



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 originally have been. They are 

 now fine flaggy micaceous gneisses and mica-schists, which cer- 

 tainly 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 condi- 

 tion 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 repre- 

 sents the upper part of the calcareous Durness series attenuated 

 and altered by the intense shearing which all the rocks have under- 

 gone. 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 times. 



Having satisfied myself that Murchison's explanation of the 

 order of sequence could not be established in Eriboll, I was desir- 

 ous 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 sections been planned for the purpose of deception they 

 could not have been more skilfully devised. The parallelism 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. But 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 deposi- 

 tion. The same kind of evidence of upthrust and metamorphosis 

 that these coast-sections reveal, can be traced southwards for a 

 distance of more than ninety miles. The task of unravelling the 

 geological structure of these southern regions will be much facili- 

 tated by the remarkable persistence of the Sutherland Silurian 

 zones, some of which, with their characteristic features and fossils, 

 are as well marked above Loch Carron as they are at Loch Eriboll. 



In South-western Ross-shire, the platform on which the Silurian 



