652 SIR A. GEIKIE AND MR. J. J. H. TEALL ON THE [Nov. 1 894, 



Unaltered olivine has been recognized only in one specimen taken 

 from a black nltrabasic ' schliere ' ; and although some of the 

 pseudomorphs of pilitic hornblende may represent this mineral in 

 other specimens, it is probable that it did not play an important 

 part in the original constitution of the rocks. The unaltered sub- 

 stance is nearly colourless in thin section, but the strings of magne- 

 tite and the yellow staining along cracks and at the edges of the 

 individual grains, described by Prof. Judd, are well developed. 

 The mineral is not idiomorphic ; but the other constituents have 

 been moulded on the rounded grains (PL XXVIII. figs. 5 & 6) in 

 such a way as to show that they must frequently be of later date. 

 No inclusions of augite in olivine have been observed. 



Magnetite (titano-magnetite) is usually present, either in the 

 form of rounded grains or as large irregular masses. Crystals are 

 rare. The mineral is found also, as we have already pointed out, 

 in the veins traversing the olivines ; but this mode of occurrence is 

 quite distinct from that with which we are now more especially 

 concerned. There can be no doubt that magnetite entered largely 

 into the original composition of these rocks. The irregular masses 

 contain rounded grains of augite as inclusions, and these are some- 

 times so abundant that the magnetite is reduced to the condition 

 of thin strings separating the augite-grains (PL XXVIII. fig. 4). 

 The analyses reveal the presence of titanic acid, and on this account 

 polished surfaces of a variety of rock composed of augite and mag- 

 netite were etched with hydrochloric acid, in order to ascertain 

 whether the iron ore was an intergrowth of magnetite and ilmenite. 

 No evidence of such intergrowth was obtained. A surface free from 

 cracks appeared to be uniformly attacked, and the solution contained 

 titanic acid. 



Brown compact hornblende is very feebly represented in these 

 rocks. It has been observed only in a few specimens, in the form 

 of small irregular patches in the augite. Green hornblende is 

 present, both in the fibrous or uralitic form and in the pilitic form. 

 Secondary hornblende and chlorite not only occur as pseudomorphs, 

 but also in narrow veins traversing the other constituents. The 

 only other mineral which remains to be noticed is epidote. This 

 has been observed only in one or two of the slides ; it occurs as 

 irregular grains in the felspar. 



The banding which forms so striking a feature of these gabbros is 

 due to a variation in the relative proportions of the four essential 

 constituents — labradorite, augite, olivine, and titano-magnetite. 

 The light-coloured bands are rich in felspar ; the dark bands are rich 

 in the ferro-magnesian constituents and magnetite. If ere and there 

 black ' schlieren/ composed entirely of augite and iron ore, occur. 

 The more basic portions are not limited to the margins of the masses, 

 but alternate with the more felspathic portions to form the banded 

 complex. There is no essential difference between the different 

 bands as regards coarseness of grain, and the individual minerals 

 interlock with each other across a junction-line just as they do in 

 the central portion of a band. It seems impossible, therefore, to 



