Chap. I.] LAUEENTIAN EOCKS OF BEITAIN. 11 
grits and conglomerates that compose the loftiest mountains in the West 
Highlands, it followed that these rocks must unquestionably represent, at 
least in great part, the Longmynd or true Cambrian rocks of South Britain 
and Wales. Now these hard grits and conglomerates are seen to repose 
abruptly on highly inclined and contorted strata of massive hornblendic 
gneiss which, unlike the overlying Cambrian and Lower Silurian strata, 
has a direction or strike nearly at right angles to them, — the former trending 
from N.W. to S.E., the latter from N.E. to S.W. The fact of the great 
divergence of strike, which I laid down on my geological map of the High- 
lands*, being coupled with the clearest evidences of infraposition to Cam- 
brian and Silurian rocks, I had no longer any doubt that these rocks were 
of a remoter age than any which had been previously recognized in the 
British Isles. Hence I at first termed them < Fundamental Gneiss,' and 
soon after, following my distinguished friend Sir W. Logan, I applied to 
them his term ' Laurentian/ and thus clearly distinguished them from the 
younger gneissic and micaceous crystalline rocks of the Central and Eastern 
Highlands f, which were classed as metamorphosed Lower Silurian. I was, 
indeed, fortified in adopting this classification by my associate Professor 
Hamsay, who accompanied me in 1859 to the Northern Highlands, and 
who not only assented to my views, but who, having himself recently 
returned from Canada, assured me that the Scottish base was unquestion- 
ably the same as that of North America. 
In the recently published and highly instructive Geological Map showing 
the distribution of the Laurentian Rocks in Northern Canada, Sir W. 
Logan and his associates £ have exhibited in separate colours no less than 
seven divisions of the Lower Laurentian, including four separate stages of 
orthoclase-gneiss and three of interstratified limestones. The Upper Lau- 
rentian or Labrador group consists mainly of anorthosite -gneiss. These 
rocks, with some syenite and porphyry and an occasional dyke of green- 
stone, occupy the whole region. 
Let us now endeavour to determine whether these Laurentian rocks of 
the Western Highlands of Scotland, which are manifestly the foundation 
stone of all the formations in the British Isles, pertain to the Lower or the 
Upper division of the North- American Laurentian System of Logan. If we 
* This great change in the estimate of the rela- 
tions and succession of the rocks in the North of 
Scotland was fully explained in memoirs commu- 
nicated to the Geological Society (Quart. Journ. 
G-eol. Soc. vols. xv. and xvi.), and illustrated by a 
geological map of the Highlands, on which the new 
classification was for the first time represented. 
| Professor ISTicol differs from me in this sepa- 
ration of the older from the younger gneiss, and, 
considering them both to be parts of the same 
series, he follows the old classification of Mac- 
Culloch. He believes that the gneiss which forms 
the low headlands of Sutherland and Ross (my 
fundamental or Laurentian rock) is brought up 
to the east of the fossiliferous Silurian limestones 
and quartz-rocks by great faults and curvatures. 
From that opinion of my former associate I en- 
tirely dissent ; and my view is sustained by every 
other geologist who has explored the country, in- 
cluding Professors Ramsay and Harkness, Colonel 
Sir H. James and Mr. Archibald Greikie. Resolving 
to look closely into the objections raised by Pro- 
fessor Nicol, I induced Mr. Greikie to accompany 
me through the north-western Highlands in 1862 ; 
and as he, after much labour, confirmed all my 
conclusions,we thereupon published the Greological 
Map of Scotland, which has been largely sold by 
Mr. A. Keith Johnstone. 
I The Canadian Greological Survey consists of 
Sir W. Logan, as Director, with his assistant Mr. 
A.Murray, Dr.T. Sterry Hunt, Chemist and Mine- 
ralogist, and Mr. E.Billings, Palaeontologist. They 
have been ably supported by Dr. Dawson, the 
Principal of M'GHll College, Montreal. 
