550 KE20RT — 1884. 



those of the 'Fundamental gneisses' all the world over — that I know 

 anything about. 



The Torridon sandstone is so obviously to a large extent made out of 

 the debris of the Hebridean (though materials of newer date may some- 

 times be observed) that it is needless to go into much detail : it may be 

 described briefly as an indurated quartz-felspar grit, varying from coarse 

 (even conglomerate or breccia) to moderately fine (grains about 0"1 inch 

 diameter) ; sometimes in small hand specimens it might readily be mis- 

 taken for a rather fine-grained granite. The ' quartzite ' varies from a 

 hard grit to an extremely consolidated quartzite that breaks with a sub- 

 conchoidal fracture, in which the individual grains seem completely ' fused 

 together.' The colour of the latter varies from a pure white to greyish or 

 yellowish or reddish tints — one quartzite being a rich liver colour ; under 

 the microscope the more compact kinds are seen to be almost wholly formed 

 of subangular grains of quartz cemented together by secondary quartz, 

 commonly in crystalline continuity with the original grain. The rock 

 was probably once a sand derived from the quartz constituent of the 

 Hebridean gneisses. The burrows of annelids abound in certain of its 

 beds, and an orthoceras has been found. The limestone varies in litholo- 

 gical character ; in some £>lfices it is fawn-coloured and dolomitic, in 

 others dark — not unlike any carbonaceous Palaeozoic limestone ; it has 

 yielded three species of orthoceras, with several other fossils. The litholo- 

 gical characters of the Eastern Gneiss have been already described. 



I am compelled, both for want of personal knowledge and of published 

 information on which I can rely, to omit the metamorphic rocks of Ireland 

 from this sketch ; but I may add that the presence of Archaean rocks, in 

 at least the north-west, is admitted, and that there is the same difficulty 

 as in Scotland of deciding whether a newer metamorphic series is to 

 be regarded as of Ordovician or of much earlier date. 



In concluding this brief sketch I may again state that, as I have 

 intimate acquaintance in the field with most of the dish'icts on which it 

 touches, and have probably examined, if I do not possess, a larger number 

 of microscopic slides from their rocks than any other person, it has been 

 impossible for me to avoid expressing my own opinion as to controversial 

 points ; but I believe that the lithological descriptions are in no respect 

 coloured by it, and are as accurate as their brevity admits, so that I hope 

 the reader may be now able to understand clearly upon what evidence 

 so many important rock-masses are transferred from the Cambrian and 

 Ordovician groups to the mysterious Archaean Period. 1 



Postscript, Sept. 1. — It may be of some little interest to mention the 

 resemblances between the above-described British rocks and those which 

 I have seen since I landed in Canada. I should hardly venture upon 

 this, seeing that I can only speak at present from a superficial and com- 

 paratively limited knowledge, but that I have had exceptional advantages 

 in being accompanied on my excursions either by Sir W. Dawson, Dr. 

 Selwyn, Dr. Girdwood, or Dr. Harrington, and am already familiar with 

 Archaean rocks in more than one other country. 



The Lower Laurentian, or Laurentian proper, wherever I have seen 

 it, closely resembles the older gneissic series of Britain, such as the 

 admitted Archaean rocks of the NW. of Scotland, and the more 

 ancient gneisses of the central Highlands — in general terms the Loch 



1 In November 1884, Dr. A. Geikie announced that the Murchisonian hypothesis 

 had been proved to be untenable. 



