[372] The Geology of the Cheviot Hills is comprised in Sheets 108 N.E., 109 N.W., and 110 S.W. of the Geological Survey of England and Wales, and in Sheets 17, 18 and 26 of the Geological Survey of Scotland. For descriptive accounts the Memoirs to some of these Sheets may be consulted, particularly "Geology of the Cheviot Hills" (English side), by C. T. Clough (Mem. Geol. Surv. 1888); "Geology of Otterburn and Elsdon," by H. Miller and C. T. Clough (Mem. Geol. Surv. 1887); "Geology of Part of Northumberland between Wooler and Coldstream," by W. Gunn and C. T. Clough, with Petrographical Notes by W. W. Watts (Mem. Geol. Surv. 1895). Other descriptions have been published by Professor James Geikie, Good Words, vol. xvii. (1876), reprinted in Fragments of Earth-lore (1893), and by Prof. Lebour, Outlines of the Geology of Northumberland, 2nd edit. 1886. For the petrography of the rocks consult Mr. J. J. H. Teall, Geol. Mag. 1883, pp. 100, 145, 252, 344; 1884, p. 226; 1885, p. 106; Proc. Geol. Assoc. ix. (1886) p. 575; and his British Petrography, 1888; Dr. J. Petersen, Mikroskopische und chemische Untersuchungen am Enstatit-porphyrit aus den Cheviot Hills, Inaugural Dissertation, Kiel, 1884.
This volcanic pile, consisting mainly of bedded andesites which rest unconformably on the upturned edges of Wenlock shales and grits, presents a most typical display of the lavas of the Lower Old Red Sandstone. These rocks range from vitreous or resinous pitchstone-like varieties to coarsely porphyritic forms, on the one hand, and to highly vesicular and amygdaloidal kinds, on the other. Analyses of some of these rocks, and an account of their petrography, have already been given.
The lavas are often separated by thin partings of tuff, and their upper surfaces show the fissured character with sandstone infillings, so characteristic among the lavas of "Lake Caledonia."[373] Tuffs form a very subordinate part of the whole volcanic series. One of the most important bands is a thick mass at the base of the series, lying immediately on the highly inclined Silurian shales. The fragments are generally of a fine-grained purple mica-andesite, often two or three feet and sometimes at least five feet long. For a few feet near the bottom of this mass of tuff, pieces of Silurian shale an inch in length may be noticed. Mr. Clough remarks that distinct bedding is not usual among the tuffs. Though no doubt most of the fragmental materials really lie intercalated between successive lava-streams, yet some of the isolated patches of coarse volcanic breccia may mark the sites of eruptive vents. One such probable neck has been mapped on the Scottish side between Cocklawfoot, at the head of the Bowmont Water, and King's Seat, while others may perhaps occur among the detached patches that have been observed on the Northumbrian side. No thick conglomerates or sandstones have been noticed in the Cheviot District. The volcanic eruptions appear to have usually succeeded each other without the spread of any notable amount of ordinary detritus over the floor of the water-basin. It is difficult to estimate the total thickness of volcanic material here piled up, but it probably amounts to several thousand feet. The top of the series is not visible, having been partly removed by denudation and partly buried under the Carboniferous formations.
[373] Clough, Geology of the Cheviot Hills, p. 15.
It will thus be seen that the Cheviot area stands apart from the other volcanic districts of the Lower Old Red Sandstone in the great relative thickness of its accumulated lavas, the comparative thinness of its tuffs, and the absence of the thick intercalations of coarse conglomerate so abundantly developed among the volcanic series all over Central Scotland. But there is yet another characteristic in which this area is pre-eminently conspicuous. In the heart of the andesites lies a core of augite-granitite, around which these rocks are traversed with dykes.
This interesting granitic boss rises into the highest summit of the whole Cheviot range, and covers an area of rather more than 20 square miles. While its petrographical characters have been described by Mr. Teall, its boundary has been mapped by Mr. Clough, who found the line difficult to trace, owing partly to the prevalent covering of peat, and partly to the jagged and irregular junction caused by the protrusion of dykes from and into the boss. He obtained evidence that the granite has broken through the bedded andesites, and that it is in turn traversed by dykes composed of a material indistinguishable from that of some of the flows. He therefore considered that it is essentially of the same age as the rest of the volcanic series, and "not improbably the deep-seated source of it."[374] Mr. Teall also, from a chemical and microscopical examination of the rocks, drew a similar conclusion.[375]
[374] Op. cit. p. 24.
[375] Geol. Mag. 1885, p. 106.
The andesites around the granite have undergone contact-metamorphism, but the nature and extent of the change have not yet been studied. There occur around the granite many dykes of felsite and quartz-felsite, to the petrographical character of which reference has already been made. But the most abundant and remarkable dykes of the district are those of a reddish mica-porphyrite, of which Mr. Clough has mapped no fewer than forty, besides those in the granitic area. He has called attention to the significant manner in which all the dykes of the district tend to point in a general way to the great core of granite, as if that were the nucleus from which they had radiated.[376]
[376] Op. cit. pp. 26-28.