Chapter XXVI. (p. 418).

Of these basic lavas conspicuous examples may be seen at Arthur Seat, Calton Hill and Craiglockhart Hill. The eastern part of Arthur Seat, known as Whinny Hill, furnishes examples of olivine-dolerites of the Jedburgh type (p. 418). The beautiful basalt of Craiglockhart with its large porphyritic olivines and augites has afforded a distinct type of Carboniferous basalt (p. 418). The same type occurs on the Calton Hill in the cliff below the gaol. Similar basic lavas are especially abundant and remarkable in the Clyde plateau near Campbeltown in Argyllshire, and at the south end of Bute and in the Cumbraes, where they are associated with an interesting series of dykes and sills. But even where, as in the Garleton Hills, the lavas are for the most part somewhat acid in composition, those first poured out, which form the lowest band, include some typical olivine-basalts, of which a characteristic example occurs at Kippie Law at the base of the Garleton plateau (p. 418). It has been described by Dr. Hatch as exhibiting under the microscope porphyritic crystals of felspar and olivine lying in a groundmass composed of lath-shaped felspars, granular olivine and magnetite, and microlitic augite. The olivine, originally the most abundant constituent, has been converted into a fibrous aggregate of serpentine. All the minerals are more or less idiomorphic, but especially the augite, which crowds the groundmass in delicately-shaped prisms, most of which are terminated at both ends by faces of the hemi-pyramid. The analysis of this rock is given in the accompanying table of analyses of Garleton basalts. The Kippie Law type of basalt was recognized by Dr. Hatch among the Geological Survey collections from other districts, as in the intrusive bosses of Neides Law and Bonchester near Jedburgh, and from the Campsie plateau a mile and a half north of Lennoxtown.[423]

[Footnote 423: _Trans. Roy. Soc. Edin._ vol. xxxvii. (1893), pp. 117-119.]

At Hailes Castle, in the Garleton plateau, the lower basic lavas include another olivine-basalt somewhat more felspathic than that just described, and at Markle quarry the rock is still more felspathic and contains the olivine only in small sporadic grains. The composition of these basic rocks of the Garleton plateau is shown in the subjoined table of analyses by Mr. J. S. Grant Wilson:--

+------------+--------------+----------------+----------------+ | | Kippie Law, | Hailes Castle, | Markle Quarry, | | | specific | specific | specific | | | gravity 2·8 | gravity 2·76 | gravity 2·7 | +------------+--------------+----------------+----------------+ |SiO_{2} | 46·01 | 49·07 | 49·54 | |Al_{2}O_{3} | 19·19 | 19·43 | 22·23 | |Fe_{2}O_{3} | 5·91 | 10·58 | 9·55 | |FeO | 6·75 | 2·35 | 1·12 | |MnO | 0·19 | 0·32 | 0·08 | |CaO | 8·68 | 7·87 | 7·19 | |MgO | 6·81 | 4·36 | 2·80 | |K_{2}O | 1·20 | 0·98 | 1·81 | |Na_{2}O | 3·27 | 3·31 | 4·56 | |H_{2}O | 3·07 | 2·26 | 2·42 | |Total | 101·08 | 100·53 | 101·30 | +------------+--------------+----------------+----------------+

Olivine-dolerites are more especially developed in the district around Jedburgh, where they form some of the most prominent bosses, such as Dunian and Black Law. They show a sub-ophitic groundmass, with inconspicuous porphyritic crystals, among which those of olivine are more prominent than the felspars (p. 418).

(_c_) Andesites (Porphyrites).--These are the most abundant lavas of the plateaux. They occur in every district, and usually form the main constituents of the pile of volcanic material. They vary in colour from a pale pinkish grey, through many shades of red, purple, brown and yellow, to sometimes a dark green or nearly black rock. Their texture ranges from almost semi-vitreous, through different degrees of compactness, to open, cellular, slaggy masses. Generally through their base porphyritic felspars are abundantly disseminated, sometimes in large, flat, tabular forms, like those of the Lower Old Red Sandstone already referred to. The amygdaloidal kernels consist of calcite, zeolites, chalcedony or quartz. It is from the amygdaloids on either side of the Clyde that the fine examples of zeolites have been chiefly obtained for which the south of Scotland has long been famed. Occasionally, as at the south end of Bute, the andesitic lavas display a marked columnar structure.

Under the microscope these rocks present the usual fine felted aggregate of felspar microlites, with granules or crystals of magnetite and sometimes pyroxene. The porphyritic felspars, often large and well defined, generally contain inclusions of the groundmass. Occasionally some of the large porphyritic constituents are augite, or pseudomorphs after that mineral. The alteration of the rocks has oxidized some of the iron-ore and given rise to the prevalent purplish and reddish tints.

(_d_) Trachytes.--Some of the most remarkable lavas to be found in any of the plateaux are those which constitute a large part of the Garleton Hills. They overlie the lower andesite and basalt platform, which surrounds them as a narrow belt, while they occupy the central and much the largest part of the area. They have been included among the porphyrites, but are pale rocks, generally with a yellowish crust, presenting when quite fresh a grey, compact, felsitic base with large porphyritic crystals of unstriped felspar.

A number of specimens selected as illustrative of the different varieties have been analyzed and the results are stated in the subjoined table.[424] The specific gravity of the rocks is about 2·6.

[Footnote 424: The first two analyses are by Mr. J. S. Grant Wilson, the last two by Mr. A. Dick jun., and that from Hopetoun Monument by Mr. G. Barrow. _Trans. Roy. Soc. Edin._ vol. xxxvii. p. 122.]

+------------+--------+--------+----------+-------------+----------+ | | Pepper | Kae | Hopetoun | Phantassie | Bangley | | | Craig | Heughs | Monument | | Quarry | +------------+--------+--------+----------+-------------+----------+ | SiO_{2} | 62·61 | 61·35 | 62·50 | 59·50 | 58·50 | | Al_{2}O_{3}| 18·17 | 16·88 | 18·51 | 18·25 | 21·12 | | Fe_{2}O_{3}| 0·32 | 0·41 | } 4·39 | 4·81 | 4·68 | | FeO | 4·25 | 5·01 | } | 2·34 | ... | | MnO | 0·21 | 0·26 | ... | ... | ... | | CaO | 2·58 | 2·39 | 2·00 | 2·10 | 3·70 | | MgO | 0·74 | 0·44 | 0·61 | 0·70 | 0·93 | | K_{2}O | 4·02 | 6·12 | 6·31 | 6·30 | 5·84 | | Na_{2}O | 6·49 | 5·26 | 3·44 | 5·03 | 3·90 | | H_{2}O | 0·80 | 1·70 | 2·10 | 1·60 | 2·00 | | Total | 100·19 | 99·82 | 99·86 | 100·63 | 100·67 | +------------+--------+--------+----------+-------------+----------+

The microscopic characters of these rocks, as worked out by Dr. Hatch, show them to be well-marked and wonderfully fresh sanidine-trachytes. Some of them are porphyritic, with large crystals of perfectly unaltered sanidine, sometimes also oligoclase. Small but well-formed crystals of yellowish-green augite, in addition to the porphyritic felspars, are imbedded in a fine groundmass composed chiefly of microlites of sanidine, but with granules of augite and magnetite plentifully interspersed, and occasionally prisms of apatite. There is a group in which the porphyritic felspars are scarce or absent. In these there is little or no ferro-magnesian constituent. Other trachytes, rather less basic than the augite-bearing varieties here referred to, occur as bosses in the Garleton Hills district, and are referred to in the following section (_e_).[425]

[Footnote 425: For fuller petrographical details consult Dr. Hatch's paper above cited.]

(_e_) Rocks of the Necks.--In the necks connected with the plateaux other types of massive rock are to be found. Among these perhaps the most frequent are trachytes, grey to pink in colour, but apt to weather yellow, exceedingly compact, sparingly porphyritic, and with a peculiar platy structure and waxy lustre. Rocks of this character also appear as sills and dykes. Other varieties that occur in similar positions are more basic in composition, including dark, coarse, granular diabases. In the Jedburgh district the most frequent rocks are beautiful varieties of olivine-dolerite and olivine-basalt, which form most of the prominent hills of the neighbourhood. These bosses are sometimes associated with agglomerates as at Rubers Law.

In the Garleton Hills district, some of the necks present another petrographical type which directly connects them with the remarkable lavas of the higher part of that plateau. Thus the rock of Traprain Law was ascertained by Dr. Hatch to be a true phonolite. In its general platy structure and sonorous ring under the hammer it reminds one of typical phonolites. Under the microscope the rock is found to consist mainly of small lath-shaped crystals of sanidine arranged in a marked minute flow-structure, but with few porphyritic crystals. It contains small crystals and ophitic patches of a light green soda-augite, with practically no magnesia in it. A small quantity of iron-ore and isolated granules of apatite are also present, together with patches of nepheline which, though generally decomposed and replaced with zeolitic products, occasionally display six- and four-sided crystal-contours. An analysis of the Traprain phonolite by Mr. Player is subjoined:--[426]

Silica 56·8 Titanic acid 0·5 Alumina 19·7 Ferric oxide 2·2 Ferrous oxide 3·5 Manganous oxide 0·2 Lime 2·2 Magnesia 0·4 Soda 4·3 Potash 7·1 Loss by ignition 2·5 ---- Spec. grav. 2·588 99·4

[Footnote 426: _Trans. Roy. Soc. Edin._ vol. xxxvii. p. 125.]

The neck of North Berwick Law was found by Dr. Hatch to be a trachyte, showing a plexus of lath-shaped sanidines that diminish in size to minute microlites, but with no porphyritic or ferro-magnesian constituent. The Bass Rock, though its geological relations are concealed by the sea, is in all probability another neck of this district. It is likewise a mass of trachyte, composed almost entirely of lath-shaped crystals of sanidine, with no ferro-magnesian constituent, but a good deal of iron ore. It shows none of the large porphyritic felspars so characteristic of the Garleton Hills lavas, but it closely resembles the non-porphyritic varieties, particularly the lavas of Score Hill, Pencraig, Lock Pit Hill, and Craigie Hill.[427]

[Footnote 427: The composition of the rocks of North Berwick Law and the Bass closely resembles that of the trachytic lavas of the plateau. For analyses, see Dr. Hatch's Paper, _ibid._ pp. 123, 124.]

(_f_) Tuffs.--The fragmentary ejections of the plateaux vary in texture from the finest-grained tuffs to coarse agglomerates.[428] As they have been derived from the explosion of andesite-lavas, they consist mainly of the debris of these rocks. They are often deep red in colour, as for example those of Dunbar, but are most frequently greenish. They have a granular texture, due to the small lapilli of various porphyrites imbedded in a fine dust of the same material. Grains of quartz, frequently to be detected even in the finer tuffs, may either have been ejected from the volcanic vents, or may have been grains of sand in the ordinary sediment of the sea-bottom. Both at the base and at the top of the plateau-series, the tuffs are interstratified with and blend into sandstones and shales, so that specimens may be collected showing a gradual passage from volcanic into non-volcanic detritus. In many of the tuffs of the necks fragments of sandstone and other stratified rocks occur, representing the strata through which the vents were drilled. In the tuffs of the Eaglesham district pieces of grey and pink granite have been met with which, if they are portions of an old granite mass below, must have come from a great depth.[429] In the coarser tuffs and agglomerates a larger variety of lava-form rocks is to be found than can be seen among the bedded lavas of the Plateaux. They include felsites and quartz-porphyries, and more rarely basic lavas (diabases, etc.).

[Footnote 428: For accounts of these rocks, see Explanation of Sheet 33 _Geol. Surv. Scot._ p. 32; Sheet 22, pp. 11-14; Sheet 31, pp. 14-17.]

[Footnote 429: Explanation of Sheet 22 _Geol. Surv. Scot._ p. 12.]