Geological Society, 377 



For Plymouth, 



i : 73a? 2 + 4*85y 2 + ±'2Sz 2 + 2-73w* 



-2(1-42^ — l-70a?s + •41^^ + 3-11^+ l'39yiv — -82zw). 



The comparison between the two sets of results, with a 

 view to ascertaining whether the discrepancy can be accounted 

 for by accident or is indicative of a real difference in species, 

 presents a problem in the theory of errors the consideration 

 of which may be deferred to another occasion. 



XL. Proceedings of Learned Societies, 



GEOLOGICAL SOCIETY. 

 [Continued from p. 230.] 



June 21st, 1893.— Dr. H. Woodward, F.B.S., Yice-President, 

 in the- Chair. 



rPTTE following communications were read : — 



-■- 1. " On Composite Dykes in Arran." By Prof. J. W. Judd, 



F.R.S., Y.P.GKS. 



It is proposed to apply the term ( composite dyke ■ to any fissure 

 which contains two or more distinct varieties of igneous rock, differ- 

 ing from one another in chemical composition or mineralogical con- 

 stitution. Such dykes, it is shown, fall into two classes : — 



(A) Dykes in which differentiation has evidently taken place in 

 the materials after their injection, as in the examples described by 

 Dr. Lawson in Canada and by Prof. Yogt in Norway. 



(B) Dykes in which we have evidence of the reopening of the 

 fissure after its first injection and the introduction of materials of 

 totally different composition. It is this class of dykes of which we 

 find such interesting illustrations in Arran. 



These Arran dykes belong to the latest volcanic eruptions of the 

 British Islands ; their analogues are found alike in the South of 

 Scotland, and in the North of England and of Ireland. They are 

 the infilled fissures along which sporadic volcanic outbursts took 

 place after the extinction of the great volcanoes of the Inner Hebrides. 

 The subaerial products of these later, and, for the most part, insigni- 

 ficant volcanic eruptions, have been all swept away by denudation, 

 except at Beinn Hiant and the Sgur of Eigg. 



The materials filling these dykes belong to two totally different 

 classes, — one distinctly basic, with about 55 per cent, of silica ; and 

 the other markedly acid in composition, with from 65 to 75 per cent, 

 of silica. The basic rock is an augite-andesite, which passes some- 

 times into an intersertal and occasionally into an ophitic dolerite 

 (tholeite and diabase) ; the glass of this rock shows a great tendency 

 to separate from the phenoerysts. The acid rock is often a highly 



Phil. Mag, 8. 5. Vol. 36. No. 221. Oct. 1893. 2 



