﻿184 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Feb. 6, 



latter rock it is full of green serpentine, some parts of the mass 

 consisting almost wholly of that mineral, which becomes less and 

 less abundant away from the limestone. The chief component mi- 

 nerals of the greater part of the porphyry are pink felspar and 

 semitransparent quartz, the latter occurring sometimes in circular 

 patches. Lastly, at least at one point, the quartz and felspar gra- 

 nules are distinctly rounded, and the rock contains numerous rounded 

 pebbles of quartz and jasper, some of which are as large and round 

 as walnuts. This same rock occurs also on the shores of Loch 

 Broom ; and the section there laid open will be immediately de- 

 scribed. 



The Upper Gneissose series does not immediately cover the lime- 

 stone, owing to the occurrence of this felspathic rock ; while the 

 junction-line is still further obscured by a thick covering of peaty 

 matter. But from the head of the gorge the shelving beds of that 

 series can be seen along the margin of Loch Auchall, having their 

 prevalent easterly dip. 



Descending the gorge of the Auchall River by the post-road, we 

 cross the admirable section of the limestone, quartz-rock, and Cam- 

 brian sandstones described by Prof. Nicol. That geologist also de- 

 tails the section along Loch Broom ; but there are some features in 

 the coast-bine which it seems ncessary to describe here. 



Pig. 5. — Section along the North side of Loch Broom. 



a. Gneiss. d. Gneissose scliists. 



b. Cambrian sandstone and conglomerate. 



c\ Quartz-rock. * Serpentinous and felspathic rock con- 



c 2 . Limestone. taining pebbles of jasper. 



Loch Broom. — The town of Ullapool stands nearly on the line of 

 demarcation between the quartz-rock and the subjacent Cambrian 

 sandstones. Following the clear natural section along the high 

 road from Ullapool to the south, we find the quartz-rock in well- 

 marked beds dipping E. 4° to 10° S., at 10° to 15°. Towards its 

 upper part the rock assumes a pink tint in alternate bands of 

 lighter and darker shades. Annelide-burrows are numerous ; and 

 it is worthy of remai-k that the tubes are white in the reddish rock, 

 recalling the aspect of those in many parts of the Carboniferous 

 rocks, where, from an under surface of shale, dark worm-pipes 

 ascend into white sandstone. Alternations of argillaceous shale 

 occur among the quartz-beds ; and at the bridge of Ault Corry we 

 meet with the limestone. Here, however, it is only about 10 or 

 12 feet thick, and dips E. 30° S., at 15° to 20°. It is covered by a 

 bed of what Professor Nicol calls " serpentine or felspar-porphyry," 

 which, in his section of Loch Broom*, is correctly represented as 

 * Quart. Journ. Geol. Soc. vol. xiii. p. 22. 



