364 On a Ternary Geological Classification. [July, 



Bbitish Fobmations abbanged under the Tebnaby 

 Classification, 



Applying the above principles to the classification of the British 

 stratagraphical series, I shall give a short statement regarding 

 each group as arranged in the accompanying table, in ascending 

 order (see Plate), reserving to a future occasion the consideration 

 of foreign groups. 



Laurentian Group (Sir W. Logan). — The existence of this 

 earliest known group of rocks in the British Isles was first 

 announced to the scientific world by Sir Roderick Murchison in 

 1856, and again in 1859. It occupies the north-west coast of 

 Scotland, from Cape Wrath southward to Loch Enard as well as 

 the whole of the Hebrides. It consists mainly of highly metamor- 

 phosed gneiss, with only thin bands of limestone at rai'e intervals. 

 The prevalent strike is from N.W. to S.E., and it is overlaid discor- 

 dantly by Cambrian rocks, with a prevalent strike from N.E. to 

 S.W.* 



The formation as represented in Britain is manifestly incomplete, 

 the calcareous beds with eozoon canadense, which are finely developed 

 in Canada, being absent, and as Sir Roderick Murchison considers 

 the British rock to be the representative of only the lower division 

 of the Laurentian series as it occurs in Canada, there is a loss of 

 one entire member. It is therefore hnpossible to represent this 

 fundamental rock in our Greological series otherwise than as a mere 

 fragment of a great formation.f 



The Cambrian Group . — This group as it occurs in Britain, 

 especially in the Longmynd, where as shown by Sir R. Murchison 

 it hes at the base of all the Silurian series, has no calcareous 

 representative. In the north-west of Scotland, the basement 

 beds, consisting of hard chocolate-coloured sandstone and con- 

 glomerate J rising into the lofty mountain of Queenaig, are truly 

 represented. These are surmounted by quartzites with crystalline 

 limestones of Lower Silurian age. In Wales, on the other hand, 

 the base is not visible; so that we find what seem to be the 

 upper beds surmounted by the " Lingula Flags " of the " Pri- 

 mordial Zone." As this great formation is devoid of a calcareous 

 representative in Britain, it cannot be regarded otherwise than as a 

 fragmentary. 



The Lower Silurian Series. — The " Lingula Flags " of Britain, 

 being entirely composed of sedimentary strata, and exhibiting no 

 very marked discordance in the stratification with reference to the 

 Llandeilo series, form naturally a portion of the lowest member of 



* See now geological map of Scotland, by Sir R. IVInrohison ami Mr. A. 

 Geikie, 18(52. 



t 'Silurin,' -Ith edition, pp. 10-12. % ll/ui., p. 15. 



