i. PETROGRAPHY OF THE GABBRO AREAS

Since the publications of Macculloch, the occurrence of beautiful varieties of highly basic rocks among the igneous masses of the Western Isles has been familiar to geologists. They were named by him "hypersthene rock" and "augite rock,"[326] names which continued in use until 1871, when my friend Professor Zirkel published the results of his tour through the West of Scotland, and showed that the rocks in question were mostly true gabbros.[327] Since his observations were published some of these rocks have formed the subject of important papers by Professor Judd.[328]

[Footnote 326: _Western Islands_, vol. i. pp. 385, 484.]

[Footnote 327: _Zeitschrift. Deutsch. Geol. Gesellsch._ xxiii. (1871), p. 1.]

[Footnote 328: _Quart. Jour. Geol. Soc._ xli. (1885), p. 354; xlii. (1886), p. 49.]

The general petrographical characters of the gabbro areas of Western Scotland may be summarized as follows:--A very considerable variety of petrological structure and chemical composition is observable among the rocks. At the one end of the series are compounds of plagioclase and augite, which, though wanting in olivine, have the general structure and habit of dolerites. At the other end are mixtures wherein felspar is scarce or absent, and where olivine becomes the chief constituent. Between these two extremes are many intermediate grades, of which the most important are those containing the variety of augite known as diallage and also olivine. These are the olivine-gabbros, which form so marked a feature in the central parts of the great basic bosses. That some of these varieties of rock pass into each other cannot be doubted. Their distinctive composition and structure appear to have been largely determined by their position in the eruptive mass. The outer and thinner sheets are in great measure dolerites, with little or no olivine. Coarse gabbros are abundant in the inner portions. Rocks rich in olivine, however, occur at the outer and especially the lower part of the gabbro masses of Rum and in some parts of Skye. The following leading varieties may be enumerated:--

Dolerite.--This rock varies from an exceedingly close grain (when it approaches and graduates into basalt) up to a coarse granular crystalline texture, in which the component minerals are distinctly visible to the naked eye. An average sample is found to consist of plagioclase, usually lath-shaped, and crystals or grains of augite with or without olivine. Under the microscope, the different varieties are distinguished by the presence of more or less distinct ophitic structure, the felspar being enveloped in the augite. For the most part they are holocrystalline, but occasionally show traces of a glassy base. Ilmenite is not infrequent, with its characteristic turbid decomposition product (leucoxene). In other cases, the iron-ore is probably magnetite. Between the dolerites and gabbros no line of demarcation can be drawn in the field, nor can a much more satisfactory limitation be made even with the aid of the microscope. As a rule, the thickest and largest intrusive masses or bosses are gabbro, those of less size are dolerite, while the smallest (and sometimes the edges of the others) assume externally the aspect of basalts.

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.

[Footnote 329: _Zeitschr. Deutsch. Geol. Gesellsch._ xxiii. (1871), p. 59.]

[Footnote 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.]

[Footnote 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]

[Footnote 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 ==== ==== ====

[Footnote 333: _Quart. Journ. Geol. Soc._ vol. 1. (1894), p. 653. Banded structures have been recognized in many gabbros of different ages. See the references in this paper; also Mr. W. S. Bayley, _Journ. Geol._ Chicago, ii. (1895), p. 814, and vol. iii. p. 1.]

(3) _Coarse-grained massive Gabbros._--These rocks, so abundant among the great basic bosses of the Inner Hebrides, are characterized by their coarse granitic structure, their component crystals being sometimes more than an inch long. They occur as sheets, veins and irregular masses traversing the varieties of gabbro already mentioned. They consist of the same minerals as the banded forms, and indeed are themselves sometimes banded. They are more uniform in composition than the typical banded gabbros, though showing also some variation in the relative proportions of their constituents. The specific gravity of three specimens was found to be 2·82, 2·97, and 3·06.

(4) _Pale Gabbros of the Veins._--These occur abundantly as irregular branching veins, from less than an inch to several yards in width, and cross all the other varieties (Fig. 330[334]). Their whiteness on weathered surfaces makes them conspicuous by contrast with the dark brown or black hue of the rocks which they traverse, and shows at once that they must be poorer in bases than these. They are found on microscopic examination to consist of the same minerals as the more coarsely crystalline gabbros, but with a much greater abundance of the felspar. They contain also apatite, and hornblende appears to predominate in them over augite. They are to be distinguished from the pale veins that form apophyses from the intrusive granophyres.

[Footnote 334: Figs. 330, 336 and 337 are from photographs taken for the Geological Survey by Mr. R. Lunn.]

Troctolite (Forellenstein).--This beautiful variety of plagioclase-olivine rock occurs as a conspicuous feature on the east side of the gabbro-area of the island of Rum. It forms a sill on the side of the mountain Allival, in which the component minerals are drawn out parallel with the upper and under surfaces of the bed (Fig. 341). So marked is this flow-structure that hand-specimens might readily be taken at the first glance for ancient schistose limestone. "The felspathic ingredient (probably labradorite or anorthite) is white, and its lath-shaped crystals have ranged themselves with their long axes parallel to the line of flow. The olivine occurs in perfectly fresh grains, which in hand-specimens have a delicate green tint. Under the microscope they appear colourless, and are penetrated by the felspar prisms in ophitic intergrowth. There is a small quantity of a pale brownish augite, which not only occurs in wedge-shaped portions between the felspars, but also as a narrow zone round the olivines."[335] Considerable differences are visible in the development of the flow-structure, and with these there appear to be accompanying variations in the microscopic structure. Dr. Hatch, to whom I submitted my specimens, informed me that in one of them, where the flow-structure is so marked as to give a finely schistose aspect to the rock, "there is a larger proportion of augite, some of which exhibits a distinct diallagic striping; the olivine grains show no ophitic structure, but are sometimes completely embedded in the augite." To this remarkable flow-structure I shall again refer in connection with the light it throws on the bedded character of much of the gabbro bosses.

[Footnote 335: MS. of Dr. Hatch.]

Between the different basic intrusive igneous rocks of the Inner Hebrides, as Professor Judd has shown, there are many gradations according to the varying proportions of the chief component minerals. Thus from the olivine-gabbros, by the diminution or disappearance of the augite we get such rocks as troctolite; where the plagioclase diminishes or vanishes, we have different forms of picrite; where the olivine is left out, we come to compounds, like eucrite; while by the lessening or disappearance of the felspar and augite, we are led to ultra basic compounds, consisting in greatest part of olivine, like lherzolite, dunite and serpentine. To some of the features and probable origin of these chemical and mineralogical diversities in the same great eruptive mass further reference will be made in later pages.