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 L XVIII. Proceedings of Learned Societies, 



GEOLOGICAL SOCIETY. 



[Continued from p. 504.] 



June 6th, 1894.— Dr. Henry Woodward, E.R.S., President, 

 in the Chair. 



THE following communications were read : — 

 1. ' On the Banded Structure of some Tertiary Gabbros in the 

 Isle of Skye.' By Sir Archibald Geikie, LL.D., D.Sc, E.R.S., E.G.S., 

 and J. J. H. Teall, Esq., M.A., F.R.S., Sec.G.S. 



After calling attention to the previous references to the pseudo- 

 bedding and banding of the gabbro-masses of the Inner Hebrides, 

 the authors describe the rocks which form the rugged ridge 

 of Druim-an-Eidhne, near the head of Glen Sligachan. This 

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

 general N.N.W. direction with a prevalent easterly dip. Four 

 distinct types of gabbro occur : — (1) dark, fine-grained, granulitic 

 gabbros ; (2) well-banded gabbros ; (3) coarse-grained massive 

 gabbros ; and (4) pale veins of a highly felspathic gabbro. The 

 relative ages of the banded and granulitic gabbros have not been 

 definitely settled; but the coarse, massive gabbros are certainly 

 intrusive in the banded series and the pale veins cut all the other 

 varieties. 



The paper deals mainly with the banded gabbros. They occur 

 in successive sheets or sills which vary from a few feet to many 

 yards in thickness, and consist of parallel layers of lighter and 

 darker material which correspond in direction with the trend of 

 the sheets, and are usually inclined to the east or south-east at 

 angles ranging from 20° to 30°. In some cases the bands can be 

 seen to have been puckered or folded. 



The minerals entering into the composition of the banded, as also 

 of the other varieties, are labradorite, pyroxene, olivine, and titani- 

 ferous magnetite. The banding is due to a variation in the relative 

 proportions of the different constituents and especially in the amount 

 of magnetite. Some narrow bands and lenticles are composed 

 entirely of pyroxene and magnetite. The variations in chemical 

 composition are illustrated by three analyses by Mr. Player. The 

 microscopic characters of the rocks are described, and it is shown 

 that the minerals of the banded gabbros have not been crushed or 

 broken since they were formed. 



The authors conclude that the banding is the result of the intru- 

 sion of a heterogeneous magma and that similar banding in certain 

 portions of the Lewisian gneiss may have been produced in the same 

 way. 



