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.
[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.
| SiO2 | Al2O3 | Fe2O3 | FeO | MnO | CaO | MgO | K2O | Na2O | H2O | Total | |
| Pepper Craig | 62·61 | 18·17 | 0·32 | 4·25 | 0·21 | 2·58 | 0·74 | 4·02 | 6·49 | 0·80 | 100·19 |
| Kae Heughs | 61·35 | 16·88 | 0·41 | 5·01 | 0·26 | 2·39 | 0·44 | 6·12 | 5·26 | 1·70 | 99·82 |
 | |||||||||||
| Hopetoun Monument | 62·50 | 18·51 | 4·39 | ... | 2·00 | 0·61 | 6·31 | 3·44 | 2·10 | 99·86 | |
| Phantassie | 59·50 | 18·25 | 4·81 | 2·34 | ... | 2·10 | 0·70 | 6·30 | 5·03 | 1·60 | 100·63 |
| Bangley Quarry | 58·50 | 21·12 | 4·68 | ... | ... | 3·70 | 0·93 | 5·84 | 3·90 | 2·00 | 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]
[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 |
[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]