.458 Britkh Association — J. ELoriie, F.R.8., etc. — 



be made also to his great paper " On the Discovery of the Olendlus Fauna 

 in the Lower Cambrian Eocks of Britain," in which he not only chronicled 

 the finding of this fauna at the top of the basal quartzite in Shropshire, 

 but suggested the correlation of the Durness quartzites and limestones 

 with the Cambrian rocks elsewhere. ^ That suggestion was strikingly 

 confirmed within three years afterwards by the discovery of the Olenellus 

 fauna in Eoss-shire. 



The detailed mapping of the belt of Cambrian strata has proved the 

 striking uniformity of the rock sequence. There is little variation in the 

 lithological characters or thicknesses of the various zones. Basal quartzites, 

 pipe-rock, Fucoid-beds, Serpulite (Salterella) grit, limestone, and dolomite 

 form the invariable sequence, for a distance of a hundred miles, to the west 

 of the line of earth-movements. This feature is also characteristic of the 

 fossiliferous zones, for the sub-zones of the pipe-rock, the Olenellus fauna in 

 the Fucoid-beds, and the Salterella limestone have been traced from EriboU 

 to Skye. Owing to the inteiTuption of the sequence by reversed faults or 

 thrusts, the higher fossiliferous limestone zones are never met with between 

 EriboU and Kishorn, but they occur in Skye, where they were first detected 

 by Sir A. Geikie.* 



Eegarding the palseontological divisions of the system, my colleague, 

 Mr. Peach, concludes "that the presence of three species of OlenelUts in 

 the Fucoid-beds and Serpulite grit of the North- West Highlands, nearly 

 allied to the American form Olenellus Tliomsoni — the type species of the 

 genus — together with Hyolitkes, Salterella, and other organisms found with 

 it, prove that these beds represent the Georgian terrane of America, which, 

 as shown by Walcott, underlies the Paradoxides zone." Hence he infers 

 that there can be no doubt of the Lower Cambrian age of the beds yielding 

 the Olenellus fauna in the North- West Highlands. Mr. Peach further 

 confirms Salter's opinion as to the American facies of the fossils obtained 

 from the higher fossiliferous zones of the Durness dolomite and limestone. 

 He states that " the latter fauna is so similar to, if not identical with, that 

 occurring in Newfoundland, Mingan Islands, and Point Levis, beneath 

 strata yielding the Phyllograptus fauna of Arenig age, that the beds must be 

 regarded as belonging to the higher divisions of the Cambrian formation." 



The intrusive igneous rocks of the Assynt region, of later date than 

 Cambrian time, and yet older than the Post-Cambrian movements, have 

 been specially studied by Mr. Teall, who has obtained results of special 

 importance from a petrological point of view. This petrographical province 

 embraces the plutonic complex of Cnoc na Sroine and Loch Borolan, and 

 the numerous sills and dykes that traverse the Cambrian and Torridonian 

 sediments, and even the underlying platform of Lewisian gneiss. He 

 infers that the plutonic rocks have been formed by the consolidation of 

 alkaline magmas rich in soda. At the one end of the series is the quartz- 

 syenite of Cnoc na Sroine, and at the other the basic augite-syenite,, 

 nepheline - syenite, and borolanite. The basic varieties occur on the 

 margin, and the acid varieties in the centre. The sills and dykes comprise 

 two well-marked types, camptonites or vogesites, and felsites with alkali 

 felspar and segirine, which he believes to represent the dyke form of the 

 magmas that gave rise to the plutonic mass.^ 



The striking feature in the geology of the North-West Highlands is the 

 evidence relating to those terrestrial movements that affected that region 

 in Post-Cambrian times, which are without a parallel in Britain. The 

 geological structures produced by these displacements are extremely com- 

 plicated, but the vast amount of evidence obtained in the course of th& 



1 Geol. Mag., Dec. Ill, Vol. V (1888), pp. 484-487. 



^ Quart. Journ. Geol. Soc, vol. xliv, p. 62. 



3 Geol. Mag., Dec. IV, Vol. VII (1900), p. 385. 



