Geological Society of London. 135 



identical with those of the Lizard, but in one case yet more distinctly 

 banded, (c) Banded gneisses sometimes rather fine-grained, variably 

 banded : quartzofelspathic layers alternating with those rich in 

 biotite or occasionally hornblende. Some of these gneisses resemble 

 the "granulitic group" of the Lizard; others recall certain of 

 the less coarse, well-banded gneisses of Scotland, e.g. south of 

 Aberdeen. Sometimes they are much " gnarled " by subsequent 

 earth-movements, by which, however, as a rule, the crystalline I'ocks 

 of the Island do not appear to have been veiy seriously affected. 

 {d) A very remarkable group of local occurrence which exhibits 

 great variety. In some places large masses of a dark green horn- 

 blende-rock are broken up and traversed by a pale red vein-granite 

 or aplite. The former rock is drawn out into irregular, lenticles, 

 elongated lumps, and finally streaks, and has been melted down 

 locally into the aplite. This then becomes a well-banded biotite 

 gneiss, which macroscopically and microscopically agrees with types 

 which are common among the Archasan rocks. Sark therefore 

 presents an example of the genesis of such a gneiss, and the authors 

 are of opinion that probably all the above-named rocks are of igneous 

 origin, but became solid ultimately under somewhat abnormal con- 

 ditions, to which the peculiar structures (which distinguish them 

 from ordinary igneous rocks) are due. They attribute the banding 

 to the effect of fluxional movements, anterior to final consolidation, 

 in a mass to some extent heterogeneous. This hypothesis they 

 consider may be applied to all gneisses or schists which exhibit 

 similar structures — that is, to a considerable number (but by no 

 means all) of the Archaean rocks. 



The second part of the paper consists of notes on some of the 

 dykes and obviously intrusive igneous rocks of the Island. Among 

 these are four (new) dykes of " mica-trap," one of which exhibits 

 a very remarkable " pisolitic " structure. The variety of picrite 

 described by Prof. Bonney in 1889 (from a boulder in Port du 

 Moulin) has also been discovered in situ. 



2. " On the Plutonic Rocks of Garabal Hill and Meall Breac." 

 By J. E. Dakyns, Esq., M.A., and J. J. H. Teall, Esq., M.A., F.R.S., 

 E.G.S. (Communicated by permission of the Director-General of 

 the Geological Survey.) 



The plutouic rocks described occur in a complex forming a belt 

 of high ground S.W. of Inverarnan, They vary considerably in 

 composition, and though gradual passages are sometimes found 

 between more or less acid rocks, at other times the junction is 

 sharp. The more acid are always found to cut through the less 

 acid when the two rocks are found in juxtaposition, and fragments 

 occurring in a rock are less acid than the rock itself. Though 

 thus shown to be of different ages, they must evidently refer to one 

 geological period. The first rocks to be formed were peridotites ; 

 then followed diorite, tonalite, granite, and eurite in order of 

 increasing acidity. 



The specific gravities, colours, and textures of the rocks are con- 

 sidered, and a detailed account of the constituent minerals given. 



