CHAP. XLIII 
THE GABBRO INTRUSIONS 
329 
Zirkel, from his examination of the Mull gab hr os, believed them to consist of 
three parts of plagioclase, two parts of olivine, and one part of diallage. 1 
Olivine, however, is not invariably present. 2 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. 3 Ilmenite (with leucoxene), 
magnetite, apatite, biotite, and epidote are not infrequent constituents. 
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. 4 
(1) Granulitie 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 granulitie 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 pseudo- 
morphs 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 arrange- 
ment 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 com- 
1 ZeitscJiv. Deutsch. Gfeol. Gesellsch. xxiii. (1871), p. 59. 
2 Professor Judd {Quart. Jour. Geol. JSoc. 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 con- 
stituent. See op. cit. vol. 1. p. 654.- 
3 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-sliapes with crystals 
or rounded grains of augite and olivine (dolerites), and thence into true basalts, magma-basalts, 
and tacliylytes {op. cit. xlii. p. 62). 
4 Quart. Journ. Gcol. Soc. vol. 1. (1894), pp. 645-659, and Plates xiii. xxvi.-xxviii. See also 
Prof. Judd’s paper, op. cit. (1886), p. 49. 
