550 



licroiiT -1SH4. 



ill 



iii< 



tliO80 of tho ' Fuudauu'utal f^'neisscs' nil tiie uoiltl over -tlml 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) tliat it is needless to go into mnch detail: it may be 

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

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

 diameter) ; sometimes in simiU hand specimens it niiglit readily bo mis- 

 taken for a rather tine-grained granite. Tho ' quartzite ' varies from a 

 hard grit to an extremely consolidated quartzite that bi'eaks witli 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 tbe more compact kinds are seen to be almost wholly formed 

 of subangnlar grains of quartz cemented together by secondary quartz, 

 commonly in crystalline (;ontinuity witli 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 ortboceras has been found. The limestone varies in litholo- 

 gical character ; in some places it is fawn-coloured and dolomitic, in 

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

 yielded three species of ortboceras, witb 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 metamoi'phic rocks of Ireland 

 from this sketch ; but I may add that the presence of Archa^■ln 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 districts on Avhich i' 

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

 of microscopic slides from their rocks than any other j)ei'son, it has been 

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

 points ; but 1 believe that the lilhological descriptions arc in no respect 

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

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

 so many important rock-masses ai'e transferred from the Cambrian ;ind 

 Ordovician groups to the mysterious Archa\an Period.' 



PosTSCitFL'T, Sept. 1. — It may be of some little interest to mention the 

 resemblances between the above-described British rocks and those whicli 

 1 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 advanta2:e> 

 in being accompanied on my excursions either b}' Sir W. DaAvsoit, Dr. 

 Selwyn, Dr. Girdwood, or Dr. Harringtcm, and am already familiar witl' 

 Archaean rocks in more than one other eountty. 



The Lower Laurentian, or Laurentian proper, wherever I have seer. 

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

 admitted Archrcan rocks of the NW, of Scotland, and tho moff 

 ancient gneisses of tho central Hifjhlands — in sxenei-al terms the Locli 



' In November 1S84, Dr. A. Geikit- announceil that the Muvcliisonian InpoUieH^ 

 had been proved t<^ be untenable. 



