56 Laurentian Rocks. 



trated by numerous veins of granite. These strata 

 much more closely resemble the Lower than the Upper 

 Laurentian rocks of Canada, and though, at so great a 

 distance from America, it is impossible to prove that they 

 are equivalent formations, the presumption that they are 

 of Laurentian age is very strong, and not the less so that 

 strata, having all the characters of Cambrian rocks, lie 

 quite unconformably upon them, fig. 54, p. 285. The 

 district was first described by Sir Roderick Murchison. 

 I can answer for the accuracy of his descriptions, having 

 inspected the ground with him, and personally mapped 

 a portion of the country at and about Durness. I know 

 of no other part of the British Islands in which Lau- 

 rentian strata certainly occur. 1 No fossils have yet 

 been observed in these British rocks. 



The CAMBRIAN and SILURIAN ROCKS of the British 

 series come next in succession. If these strata were 

 to be classified for the first time in England, with my 

 present knowledge, I would divide them into three, as 

 the most convenient method. The first series would 

 include the purple and green grits and slates of the 

 Longmynd and Wales, and range upward as high as 

 the top of the Tremadoc slates ; the second would range 

 from the base of the Arenig slates to the top of the 

 Bala or Caradoc beds, and the third from the base of 

 the Upper Llandovery beds to the top of the Ludlow 

 rocks. In Wales and its neighbourhood, where the 

 most typical series is found, each of these great bound- 

 ary lines is marked by unconformity, and analogous 

 unconformities are more or less found elsewhere in the 

 British Islands. 



1 After their discovery by Murchison, the Laurentian rocks and 

 other details in the Highlands were mapped by Professor Geikie, 

 F.R.S. See his Geological Map of Scotland, 1876. 



