Siluria : a history of the oldest rocks in the British Isles and other countries with sketches of the origin and distribution of native gold, the general succession of geological formations, and changes of the earth's surface / by Sir Roderick Impey Murchison.
- Roderick Murchison, 1st Baronet
- Date:
- 1867
Licence: Public Domain Mark
Credit: Siluria : a history of the oldest rocks in the British Isles and other countries with sketches of the origin and distribution of native gold, the general succession of geological formations, and changes of the earth's surface / by Sir Roderick Impey Murchison. Source: Wellcome Collection.
41/774 page 11
![f Chap. L] 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 tho 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 t, which were classed as metamorphosed Lower Silurian. I was, indeed, fortified in adopting this classification by my associate Professor llamsay, 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 Hocks in Northern Canada, Sir W. Logan and his associates t have exhibited in separate colom’s no less than seven divisions of the Lower Laurentian, including four sejmrate stages of orthoclase-gneiss and three of intorstratified limestones. The Upper Lau- rentian or Labrador group consists mainly of anorthosite-gneiss. These rocks, with some tsyenite and porphyry and an occasional dyke of green- stone, occupy the whole region. Let us now endeavour to determine whether these Laurentian rocks of tho Western Highlands of Scotland, which arc manifestly tho 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 e?tiraate of the rela- tions and succession of the rocks in the North of Scotland was fully explained in memoirs commu- nicated tortile Geological SocieW (Quart. Journ. Geol. Boc. Tols. xv. and xvi.), and illustrated by a geological map of the Highlands, on which the new classitication was for the first time represented. t Professor Nicol 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 follow's the old classification of Mac- Culloch. He believes that the gneiss which forms the low headlands of Sutherland and lioss (my fundamental or Laurentian rock) is brouglit up to the east of the fossiliferous Silurian limestones and quartz-rocks by great faults and eurvaitures. From that opinion oi my former associate I en- tirely dissent; and my view is sustained by every other geologist who has explored the countr}', in- cluding Professors Pamsay and Harkness, Colonel Sir H. James and Mr. Archibald Geikie. Pesolving to look closelv into the objections raised by Pro- fessor Nieol, 1 induced Mr. Geikie to accompany me through the north-western Highlands in 1862 ; and as he, after much labour, confirmed all my conclusions,wc thereupon published the Geological Map of Scotland, m Inch has been largely sola by Mr. A. Keith Johnstone. I The Canadian Geological Survey consists of Sir W. Logan, as Director, with his assistant Mr. A.Murray, Dr.T. Sterrv Hunt. Chemist ami Mine- ralogist, and Mr. E. Hillings. Palaeontologist. They have been ably supported bv Dr. Dawson, the Principal of M'Gill College, Montreal.](https://iiif.wellcomecollection.org/image/b28094360_0041.jp2/full/800%2C/0/default.jpg)
