314 DE. r. DixET ON THE [vol. Ixxviii^ 



single specimen of the rock shows : — Sihca 47*96 per cent., alumina 

 12'78, iron-oxides 11-19, and magnesia 16'58. This rock is a 

 variety of beerhachite (Chelius), defined by H. Kosenbusch^ a& 

 a fine-grained to compact granular rock, composed of labradorite 

 (sometimes bytownite) and diallage, with variable amounts of 

 hyjDersthene and magnetite ; he states also that olivine-bearing 

 varieties occur. Similar rocks are figured by Dr. J. S. Flett 

 in the Lizard Memoir, and by Kosenbusch in his ' Elements der 

 Gesteinslehre.' ^ The rock described by the former, referred to 

 again beloAV, occurs in the form of d3dves in the Lizard gabbro, and. 

 consists mainly of granular labradorite and augite.^ Kosenbusch 

 states ^ that beerbachites occur in the gabbros of Bum and Kilhoau' 

 (Ardnamurchan) and in that near Harzburg. Similar rocks occur 

 also among the Skj^e gabbros, and are described as ' granulitic 

 gabbros ' or ' pyroxene-granulites.' ^ 



The ' fine-grained granulitic gabbro ' of the Duluth laccolite ^ 

 so closel}^ resembles the Sierra-Leone beerhachite that microphoto- 

 graphs of the two rocks can scarcely be distinguished. This 

 granulitic gabbro, like the beerhachite now considered, is both 

 darker and denser than the rock of the main intrusion. 



J. W. Judd^ has explained the texture of certain Tertiary 

 granulitic gabbros as due to crystallization of a moving magma in, 

 which the grains as they were formed Avere rolied around and 

 prevented from interlocking freely. The Sierra-Leone beerhachite 

 evidently crystallized under somewhat similar conditions, because 

 fragments of norite caught up in it Avere softened, disintegrated, 

 and drawn out into streaks (see below, p. 315). On the other 

 hand, a few coarsely-crystallized and fresh-looking patches, free 

 from obvious signs of contact-metamorphism, probably represent 

 portions of the magma which crystallized under static conditions. s- 



With further reference to the Lizard beerhachite, it is evident, 

 from some interesting notes recently published by Prof. T. Gr.. 

 Bonney,^ that this rock is ver}^ closely paralleled in occurrence and 

 field-characters by the Sierra-Leone beerhachite. For instance, the 

 Sierra-Leone rock also is frequently ' brown-speckled' on a weathered 

 surface, and it occurs in small masses rather than as veins ; it 

 invades a gabbro-mass made up of separate intrusions of different 

 textures, and is followed by a series of clolerite-dykes. But the 

 closest parallel of all appears in the way in which the Sierra-Leone 



1 ' Elemente der Gesteinslehre ' 3rd ed. (1910) p. 282. 



'^ Ibid, %. 6, p. 57. 



'■^ 'The Geology of the Lizard & Meneage ' Mem, Geol. Surv. 1912, p. 110 

 & pi. ix, fig. 5. 



^ 'Elemente der Gesteinslehre ' 3rd ed. (1910) p. 283. 



^ ' The Tertiary Igneous Eocks of Skye ' Mem. Geol. Surr. 1904, p. 115. 



6 M. L. Nebel, Econ. Geol. vol. xiv (1919) p. 379 ; also pi. xiv, fig. d. 



■^ ' On the Gabbros, Dolerites, & Basalts of Tertiary Age in Scotland & 

 Ireland ' Q. J. G. S. vol. xlii (1886) p. 49. 



^ Compare the coarse-grained rock cutting the Duluth granulitic gabbro : 

 see M. L. Nebel, op. supra cit. p. 381. 



9 ' Beerhachite at the Lizard ' Geol. Mag. 1920, p. 339. 



