Vol. 50.] BANDED STRUCTURE OF SOME TERTIARY GABBROS. 647 



discussing the various structures presented by the rocks of this 

 locality, we are brought face to face, not with marginal phenomena 

 of protrusion, but with the conditions under which the igneous 

 material was forced upward and consolidated in the deeper parts of 

 a great vent or duct. 



]N T o better example could be cited of the remarkably complex 

 nature of the great gabbro areas of the Inner Hebrides than is 

 furnished by Druim an Eidhne. This ridge, though forming part 

 of the central portion of one of these areas, does not consist of 

 merely one type of rock, belonging to one period of extrusion. It 

 is made up mainly of parallel beds, sills, or sheets, disposed in a 

 general ST.N.W. direction with a prevalent easterly dip. These, 

 though presenting considerable differences in texture, minute 

 structure, and composition, may be classed under the common 

 designation of ' gabbro.' Along their eastern edge a considerable 

 mass of coarse agglomerate may mark the position of one of the 

 older vents which served as passages for the uprise of the basic 

 eruptive rocks. 



Pour distinct varieties of material may be recognized among the 

 beds : (1) dark, fine-grained, granulitic gabbros which resemble 

 externally basalt-rocks ; (2) well-banded gabbros, composed of 

 irregularly alternating bands and laminae of the several constituent 

 minerals ; (3) coarse massive gabbros destitute of any banding ; 

 and (4) pale felspathic veins. 



The sequence of these varieties can to a certain extent be 

 definitely established. It was not satisfactorily ascertained whether 

 the first-named series was anterior or posterior to the second. But 

 there can be no doubt that the coarse massive gabbros (3) have 

 sometimes been injected into both of them. It is equally certain 

 that the light-coloured veins are the latest of all, for they traverse 

 all the three other varieties. 



1. The dark, fine-grained Granulitic Gabbros. — These rocks play a 

 subordinate part in the structure of the ridge. In some parts they 

 weather with a smooth brown surface, on which their minute 

 reticulated veinings of epidote and calcite stand out prominently. 

 Below the outer skin a thin white crust may often be observed. 

 Occasionally small oval patches may be noticed, like half-effaced 

 amygdaloidal kernels. 1 These external features of resemblance to 

 some of the altered conditions of the plateau-lavas are, however, 

 probably deceptive, for, as will be shown, the internal structure of 

 the rocks does not connect them with these lavas. 



That these dark fine-grained sheets are older than much of the 

 rest of the gabbro of the ridge is well shown by the way in which 

 the coarser varieties invade them and enclose lenticular patches and 

 blocks of them. But they run parallel with the banded sheets, and 

 though they might be presumed to be older than the latter we were 

 unable to obtain convincing proof of this relation. 



1 Trans. Boy. Soc. Edin. vol. xxxv. pt. i. (1888) p. 169. 



