Vol. 50.] BANDED STETTCTTjEE OF SOME TEETIAEY GABBEOS. 645 



40. On the Banded Steuctuee of some Tebtiaey Gabbeos in the 

 Isle o/Seye. By Sir Aechibald Geikie, D.Sc, LL.D., F.R.S., 

 F.G.S., and J. J. H. Teall, Esq., M.A., F.B.S., Sec. G.S. (Read 

 June 6th, 1894.) 



[Plates XXVI.- XXVIII.] 



Contents. 



Page 

 Introduction 645 



I. General Arrangement and External Characters of the Rocks 646 



II. Microscopical and Chemical Characters of the Rocks 650 



III. General Deductions 655 



(i) As regards the Conditions of the Protrusion of Igneous Magmas; 

 (ii) As regards the Structure and Origin of Ancient Gneisses. 



InTEODUCTIOJST. 



The dark basic rocks which stand out so prominently in the 

 geological structure and scenery of the Inner Hebrides have long 

 attracted notice and have been the subject of frequent description. 

 The writings of Macculloch, 1 Yon Oeynhausen and Von Dechen,' 2 

 and J. D. Forbes 3 in the earlier half of this century, and of 

 Prof. Zirkel 4 and Prof. Judd D in the later half, have made geologists 

 familiar with the general character and distribution of the Tertiary 

 gabbros. There is one feature, however, in these interesting masses 

 which has hardly received as yet the attention which it deserves. 

 "We refer to the frequent bedding and banding which they present. 

 Even from a distance this structure may be distinctly recognized on 

 many of the great declivities. The northern hills of Bum, for 

 instance, can be readily seen to be built up of successive sheets, and 

 on the flanks of the Cuillin Hills in Skye a similar structure may be 

 observed. Macculloch has referred to what he calls the " obscurely 

 bedded disposition " of some of these rocks, and Prof. Judd has 

 stated that " the great masses of gabbro in Bum exhibit that pseudo- 

 stratification so often observed in igneous rocks." One of the 

 authors of the present paper has drawn particular attention to this 

 structure as evincing that the gabbro masses are not simple eruptive 

 bosses, but are composed of many sills and sheets, the result of 

 successive protrusions of material. 6 He has also dwelt on the fact 

 that not only do these masses exhibit a bedded arrangement of 

 their materials, but that their individual beds sometimes display a 



1 ' Western Islands,' 1819, vol. i. 



2 Karsten's Archiv, vol. i. p. 99. 



3 Edinburgh New Phil. Journ. vol. xl. (1846) p. 85. 



4 'Geologische Skizzen von der Westkiiste Schottlands,' Zeitschr. deutsch. 

 geol. Gesellsch. vol. xxiii. (1871). 



3 Quart. Journ. Geol. Soc. vols. xxx. (1874) p. 220 ; xli. (1885) p. 354 ; xlii. 

 (1886) p. 49. See also the memoir by A. Geikie in Trans. Roy. Soc. Edin. 

 vol. xxxv. pt. i. (1888) p. 21. 



6 A. Geikie, Trans. Roy. Soc. Edin. vol. xxxv. pt. i. (1888) pp. 124-143. 



2y2 



