Gabbro.—Under this term I arrange, as proposed by Professor Judd, all the coarse-grained granitoid basic rocks of the region without reference to the variety of augite present in them. Under the microscope, they are found to be holocrystalline, but with a granitic or granulitic rather than an ophitic structure, though traces of the latter are by no means rare. To the naked eye their component minerals are usually recognizable. Professor Zirkel, from his examination of the Mull gabbros, believed them to consist of three parts of plagioclase, two parts of olivine, and one part of diallage.[329] Olivine, however, is not invariably present.[330] The pyroxene also does not always show the peculiar fibrous structure of diallage. Professor Judd, indeed, maintains that the diallagic form is due to a deep-seated process of alteration (schillerization), and that the same crystal may consist partly of ordinary augite and partly of diallage.[331] Ilmenite (with leucoxene), magnetite, apatite, biotite, and epidote are not infrequent constituents.
[329] Zeitschr. Deutsch. Geol. Gesellsch. xxiii. (1871), p. 59.
[330] Professor Judd (Quart. Jour. Geol. Soc. xlii. p. 62) believes that originally all the gabbros contained olivine, and that where it is now absent, it has been altered into magnetite or serpentine. But in some coarse massive gabbros this mineral does not appear to have been an essential constituent. See op. cit. vol. l. p. 654.
[331] Op. cit. xli. In a later paper he insists on the gradation of the coarse granitoid varieties (gabbros) into holocrystalline compounds, where the felspar appears in lath-shapes with crystals or rounded grains of augite and olivine (dolerites), and thence into true basalts, magma-basalts, and tachylytes (op. cit. xlii. p. 62).
In a recent study of the gabbros of the Cuillin Hills of Skye by Mr. J. J. H. Teall and myself, four characteristic types have been recognized.[332]
[332] Quart. Journ. Geol. Soc. vol. 1. (1894), pp. 645-659, and Plates xiii. xxvi.-xxviii. See also Prof. Judd's paper, op. cit. (1886), p. 49.
(1) Granulitic Gabbros.—These are dark, fine-grained rocks which externally resemble some of the altered basalts of the plateau-series. They occur in bands or sheets which, so far as can be made out, are the oldest portions of the whole gabbro mass. Under the microscope they are found to possess a finely granulitic structure, and to consist of grains of pyroxene (augite, but more usually with the inclusions characteristic of diallage and pseudo-hypersthene), and of felspar allied to labradorite, with green pseudomorphs agreeing in form and size with the pyroxene-grains, but made of minute prisms and fibres of green hornblende and a little chlorite.
(2) Banded Gabbros.—These are characterized by a remarkable arrangement in parallel bands of different mineral composition like the banding of ancient gneisses. This structure will be more particularly described in later pages. They are coarse-grained rocks composed of pyroxene, plagioclase, olivine and magnetite. But these minerals are not distributed equally through the mass. The pale bands contain much felspar; the dark bands are largely composed of the ferro-magnesian minerals and magnetite. The pyroxene, occurring as ordinary augite, not uncommonly shows a tendency to ophitic structure. The felspar, a variety closely allied to labradorite, occurs as grains, as irregular ophitic patches, and also in forms that give broad rectangular sections. Olivine in an unaltered condition has been detected by Mr. Teall in only one specimen, and he thinks that this mineral probably never played an important part in the original constitution of these rocks. Its rounded grains may be observed to have the other minerals moulded round them, whence it may be inferred to be of older consolidation. Magnetite is generally present, either in rounded grains or in large irregular masses. Though it occurs also in strings traversing the other minerals as a secondary product, it must undoubtedly have entered largely into the original composition of these rocks. It is found enclosing the augite grains and behaving like a groundmass between the felspars. Among the dark bands there occur narrow lenticular black layers ('schlieren') composed entirely of augite and iron-ore.
The extraordinary differences between the composition of the pale felspathic and the dark ultra-basic bands are well brought out in the following analyses by Mr. J. Hort Player, No. 1 being from a light-coloured band consisting mainly of labradorite with some augite, uralitic hornblende and magnetite; No. 2 from a dark band composed of augite, magnetite and labradorite; and No. 3 from a thin ultra-basic layer mainly formed of augite and magnetite. All these specimens were taken from the ridge of Druim an Eidhne, on the eastern side of the Cuillin Hills, Skye.[333]
| I. | II. | III. | |||
| Silica | 52·8 | 40·2 | 29·5 | ||
| Titanic acid | ·5 | 4·7 | 9·2 | ||
| Alumina | 17·8 | 9·5 | 3·8 | ||
| Ferric oxide | 1·2 | 9·7 | 17·8 | ||
| Ferrous oxide | 4·8 | 12·2 | 18·2 | ||
| Ferric sulphide | ··· | ·4 | ·4 | ||
| Oxide of manganese | ··· | ·4 | ·3 | ||
| Lime | 12·9 | 13·1 | 10·0 | ||
| Magnesia | 4·8 | 8·0 | 8·7 | ||
| Soda | 3·0 | ·8 | ·2 | ||
| Potash | ·5 | ·2 | ·1 | ||
| Loss by ignition | 1·2 | ·5 | 1·0 | ||
| 99·5 | 99·7 | 99·2 | |||
| Spec. grav. | 2·91 | 3·36 | 3·87 |