IN THE NORTH-WEST HIGHLANDS OF SCOTLAND. 409 



The thickness of these dykes and sheets varies from 10 to 50 feet. 

 But towards the southern limits of Assynt there is quite an ex- 

 ceptional development of one of these intrusive masses extending 

 continuously for a distance of five miles from Ledbeg to a point near 

 the road leading to Loch Ailsh. Occurring within the area affected 

 by the Post-Lower-Silurian movements, it is traversed by numerous 

 thrusts which have had the effect of repeating portions of the mass. 

 It is highly probable that it resembles the others in its mode of oc- 

 currence, and was originally injected along the bcdding-i)lanes, 

 forming a sheet not less than 600 feet thick. As might naturally 

 be expected, such a great thickness of igneous material has developed 

 important changes in the lithological characters of the strata by con- 

 tac t-metam orphism . 



3. Area of Distribution. 



The area of undisturbed Lower Palaeozoic strata penetrated by 

 these sheets is limited, extending from Loch Assynt to near Elphin, 

 a distance of about nine miles ; but their development in the terri- 

 tory affected by the Post-Lower-Silurian movements is somewhat 

 remarkable. Indeed from the evidence obtained by the detailed 

 mapping of the displaced masses, it is clear that originally the 

 igneous rocks must have spread over a large area, stretching from 

 Glencoul to Ullapool — a distance of twenty-four miles. In the 

 latter district they occur near the base of the limestone or between 

 the limestone and the Serpulite-grit ; while in Strath Kauaird (north 

 of Ullapool) they are met with apparently about the junction of the 

 " pipe-rock " with the basal quartzites. It is further evident that 

 they must have extended far to the east (though how far it is im- 

 possible to say), because they occur in the Cambrian sandstones and 

 Silurian strata carried westward along the higher thrust-planes. 



4. MacroscopiG Characters. 



A detailed description of the petrographical characters of these 

 igneous rocks is not contemplated in this paper. It will be sufficient 

 if we indicate their general macroscopic characters and the types to 

 which they belong. In his papers published in the ' Mineralogical 

 Magazine'*, Dr. Heddle has called attention to some of their litho- 

 logical features, and has figured some of the beautiful crystals of 

 felspar and hornblende ; Professor Bonney t has described the mi- 

 croscopic characters of certain specimens from the Traligill burn, 

 Inchnadamfi", and near AJltnacallagach, naming the former a 

 hornblendic porphyrite ; while Mr. Teall J has contributed notes on 

 the macroscopic and microscopic characters of several dykes taken 

 from diff'erent horizons in the Silurian series, giving the chemical 

 analyses of three varieties, which show a considerable variation in 

 chemical composition, the most basic being obtained from the series 



* Mineralog. Mag. toI. iv. p. 233, vol. v. pp. 137-145, 

 t Quart. Journ. Geol. Soc. vol. xxxix. p. 419. 

 X Geol. Mag. dec. 3, vol. iii. p. 346. 



