Vol. 50.] BANDED STRUCTURE OF SOME TERTIARY GABBROS. 653 



account for the banding on the supposition that magmas of varying 

 composition have been successively injected. The individual minerals 

 are optically perfect, and their mutual relations are such as occur in 

 igneous rocks. Cataclastic phenomena have not been observed in 

 any of the slides prepared from the banded series, and we therefore 

 conclude that the cause which produced the banding must have 

 operated before the crystallization of the minerals from an igneous 

 magma. 



Some idea of the variability in composition of different layers 

 may be formed from the following analyses kindly made for us by 

 Mr. J. Hort Player in the Laboratory of the Survey at the Museum 

 of Practical Geology, Jermyn Street : — 



Analysis of I. II. III. 



Silica 52-8 



Titanic acid '5 



Alumina 178 



Ferric oxide 1*2 



Ferrous oxide 4 - 8 



Ferric sulphide 



Oxide of manganese 



Lime 12-9 



Magnesia 4 - 8 



Soda 3-0 



Potash - 5 



Loss by ignition 1*2 



99-5 99-7 99-2 



40-2 



29-5 



4-7 



9-2 



9-5 



3-8 



9-7 



17-8 



122 



18-2 



•4 



•4 



•4 



•3 



131 



10-0 



8-0 



8-7 



•8 



•2 



•2 



•1 



•5 



1-0 



Sp. Gr 2-91 3-36 3-87 



J. Hort Plates, April 9th, 1894. 



I. (5373 x ). A light-coloured band mainly composed of labradorite. 

 The other constituents are augite, uralitic hornblende, and 

 magnetite. (See PL XXVIII. fig. 1.) 

 II. (5377). A dark band composed of augite, magnetite, and 



labradorite. (See PI. XXVIII. fig. 3.) 

 III. (5376). A thin ultrabasic ' schliere,' mainly composed of 

 augite and magnetite. (See PI. XXVIII. fig. 4.) 



It is evident from these facts that rock-specimens varying greatly 

 in chemical and mineralogical character may be obtained from the 

 banded gabbros of Druim an Eidhne. Some may be termed normal 

 gabbros, others magnetite-gabbros ; while others again consist of 

 magnetite and pyroxene, and may be called magnetite-pyroxenites. 

 These last, however, so far as our observations go, are not developed 

 on any large scale. 



Occurrences similar to these are known in many other localities 

 where gabbros, norites, and hyperites are developed on a large scale, 



1 The numbers are those of the microscopic slides in the Collection of the 

 Geological Survey. 



