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Published online by Cambridge University Press: 17 January 2013
In the Geology of Scotland, no feature comes at once more prominently before the observer than the number and variety of the trap-rocks. From the Sheant Isles to the Cheviot Hills the map is dotted over with trappean patches, which occur sometimes as long, narrow dykes, and often as irregular sheets, that extend almost over entire counties. The appearance of the rocks on the face of the country is scarcely less marked than on the map, and to their ever-changing varieties, we owe not a little of the characteristic scenery of Scotland. Throughout the wilds of the Inner Hebrides it is the trap-rocks which form many of the precipitous cliff-lines, and the craggy irregular hills. The soft pastoral valleys of the Ochils and the Pentlands lie among trappean rocks, while the hill-ranges, which break up the great central valley of Scotland, more especially the abrupt solitary crags that form such prominent landmarks, owe their existence to the permanence of the trap-rocks of which they consist. In short, there is no group of rocks more constantly found throughout the length and breadth of the island, and none, therefore, which so frequently obtrudes on the geologist or more imperatively demands his attention.
page 633 note * The word trap is here used in a generic, not in a specific sense. It includes greenstones, basalts, felstones, ashes, &c; in short, all the truly igneous rocks of former geological periods, whether volcanic (erupted at the surface) or intruded as dykes and irregular masses into other rocks. Granite, serpentine, and the granitoid porphyries I regard as not truly igneous, and they are therefore excluded.
page 634 note * In the colouring of the maps of the Geological Survey, this arrangement is followed, by using crimson for the augitic, and a brighter red for the felspathic traps. The dark shade of each colour marks the intrusive masses, while the lighter shade is used for the interbedded sheets. The age of the latter, as well as of the ash-beds, is always shown by the geological position of the strata with which they are associated.
page 636 note * Geology of Fife and the Lothians. Edinburgh, 1839.
page 637 note * Since the earlier part of this paper was written, I have had an opportunity of examining the geology of the east part of Berwickshire; especially the great ashy series in the neighhourhood of Reston, and along the coast from near Coldingham to Eyemouth. The structure of this region is often very obscure. It is possible that some of the felspathic conglomerates of the Ale Water may belong to the Lower Silurian series, but the greater part, if not the whole, of the ashy rocks of this district must, I think, be regarded as of Lower Old Red Sandstone age. Near the village of Reston I have found, in some shales in the ashy strata, remains of plants and broken crustacean fragments, which my colleague, Mr Salter, is inclined to consider as belonging to the Pterygotus. These strata are covered unconformably by the Upper Old Red Sandstone.
page 638 note * See Quart. Jour. Geol. Soc., vol. xvi. p. 312.
page 639 note * How far the Middle Old Red is represented in the great series of purplish sandstones of Lanarkshire cannot at present be determined. It would probably be nearest the truth, however, to regard these felspathic dykes as of Middle Old Red Sandstone age.
page 642 note * Transactions of the Geological Society of Glasgow, Part I., 1860.
page 643 note * See Mr Maclaren's “Sketch of the Geology of Fife and the Lothians,” p. 124, and “The Geology of Edinburgh,” in the Memoirs of the Geological Survey, where I have described the hills in detail. Compare also the Survey Map of this district, sheet 32 (Scotland).
page 643 note † See Memoir to accompanying sheet, 32 of Geol. Survey of Scotland.
page 644 note * Geol. of Fife and Lothians, p. 183. In this volume the structure of the Pentland Hills is delineated with p-reat clearness, and in a truly philosophical spirit.
page 645 note * For the details of part of this region, see the Memoir of the Geological Survey on the “Geology of Edinburgh” already referred to.
page 647 note * A beautiful example occurs in the East Quarry, Kirkton, first noticed by Dr Hibbert (Trans. Royal Soc., Edinburgh, vol. xiii. p. 278). See also “Geology of Edinburgh” (Memoirs. Geol. Survey).
page 648 note * Quart. Jour. Geol. Soc., vol. xiv.
page 649 note * Quart. Jour. Geol. Soc., vol. xiv. p. 1, et seq.
page 649 note * Essai Geologique sur l'Ecosse, p. 220, et seq.
page 650 note * For a full statement of the evidence on which this conclusion is based, see “Geology of Edinburgh” (Mem. Geol. Surv.).
page 651 note * On the Geology of Roxburghshire.—Trans. Roy. Soc. Edinb., vol. xv. p. 456.
page 651 note † In these islands there is an older group of rnagnesian greenstones, which run parallel to the strike of the schists—i.e., from south-west to north-east. It is instructive to see how completely these are cut through by the newer group. I have described these features in a joint paper on the Metamorphic Rocks of the Highlands, by Sir Roderick Murchison and myself (Quart. Jour. Geol. Soc., vol. xvii p. 210). The same facts have been observed by Mr Jameson in Cantyre and Knapdale (Quart. Jour. Geol. Soc., vol. xvii. p. 140).
page 651 note ‡ See my paper on Skye and Pabba (Quart. Jour. Geol. Soc. vol. xiv. p. 1, et seq).
