BOCKS OP THE NORTHERN HIGHLANDS. 411 



V. General Considerations. 



If the Eastern Gneiss is older than the Assynt series, it must be 

 Archaean. Whether or not the Assynt dolomite is the equivalent 

 of the Durness limestone, it is not of less antiquity ; so that the 

 gneiss is at once proved to be at least Pre-Ordovician. But it can 

 hardly be Cambrian, for we have no evidence in the British Isles of 

 any metamorphic rocks of that age. 



But Murchison extended his induction to nearly all the meta- 

 morphic rocks of Scotland outside of the Hebiidean area; and 

 Harkness, supported by Murchison, applied it to the schistose rocks 

 of Ireland. These wide-reaching conclusions, should my evidence 

 be accepted, must also fall to the ground. 



It remains to inquire to which of the Archaean groups must the 

 Eastern Gneiss be referred. Dr. Sterry Hunt claims it for his 

 Montalban group. This view can, I presume, be affirmed or rejected 

 by no one; and our consideration of it must be postponed till our 

 knowledge of these old rocks is riper. There are some lithological 

 resemblances between the Eastern Gneiss and the older of xhe 

 Anglesey Archaean series, — between, for example, the flaggy quart- 

 zose gneiss and the green chloritic schists of both areas. If we are 

 able hereafter to connect the two formations, it will probably be 

 done by taking Ireland as a stepping-stone. At present it appears 

 to me the safest course to adopt a local description ; and, from the 

 wide extent of the gneiss throughout the ancient Caledonia, I have 

 proposed the term " Caledonian." 



I do not assume that all the newer gneissic rocks of Scotland 

 belong to this series. Dr. Hicks claims to have identified several 

 groups south of Loch Broom ; but I am unable to express an opinion 

 of his work from personal observation. 



My conclusions will remove a theoretical difficulty which has 

 doubtless burdened the minds of many geologists. It has been 

 regarded as highly perplexing that comparatively unaltered rocks 

 should be overlain by a metamorphic series. How could metamor- 

 phism have taken place in an upper series without affecting the 

 lower, except on some principle of selective change ? But this ex- 

 planation cannot here apply, since parts of the Assynt series are 

 composed of materials which could be readily decomposed and re- 

 composed. 



I have more than once expressed the opinion that, so far as 

 England and Wales are concerned, all areas which had undergone 

 regional metamorphism were Archaean. These views seemed to me 

 to be confirmed by my study of the rocks of Leinster. But a much 

 more extensive territory must now be annexed to the Archaean king- 

 dom, and we seem to be approaching the induction that all British 

 metamorphic districts are included within its boundaries. This will 

 greatly add to the importance and interest of our Archaean studies. 



It will perhaps assist in anticipating certain objections if I de- 

 scribe the sequence of events which, in my view, has taken place 

 in North-western Scotland. 



