254 Dr. J. A. Smythe — Inclusions in Great WJdn Sill. 



the inclusions represent a primary crystallization of the magma, 

 consisting, as they do, so largely of felspar, closely allied in com- 

 position to that of the Whin Sill (from which it is the first mineral 

 to crystallize in quantity), and abounding in apatite, one of the 

 earliest of the minerals to separate from the magma. This case, 

 therefore, resembles in some degree that whereby the anorthite 

 inclusions of the Crookdene group of local dykes were formed.^ Both 

 are products of early crystallization, but whereas the anorthite in the 

 dykes crystallized under plutonic conditions and was floated up in 

 a solid state, the inclusions evidently came into position in a liquid 

 form, or at most with the iron minerals consolidated. Differentiation 

 in the latter case has thus been effected either in the liquid magma, 

 or an early solid product of the process has been remelted owing to 

 some change in condition. 



The interpretation of the salient characteristics of the inclusions, 

 tIz. the facetting and crystallization-phenomena, presents many 

 difficulties and uncertainties. The beautifully marked surfaces are 

 clearly the result of the crystallization of material, mainly felspathic, 

 in contact with the plane surfaces of the films, and the proof from 

 the thin sections that this material lies on both sides of the film 

 explains the similarity in appearance of both knobs and moulds. 

 The corollary to this view is that the pseudo-crystal form of the 

 inclusions is determined by the arrangement of the film-plates in 

 a liquid medium. 



If full credit be given to observations made in the field and 

 laboratory, that only the lower hemisphere of an inclusion in its 

 natural position is facetted, then one of the formative factors would 

 appear to be the settling, under gravity, of the heavy already 

 fashioned films of iron minerals in a molten matrix. The only other 

 recognizable directive force which may have been operative in 

 arranging the plates is magnetic ; for, as already stated, the films 

 show well-defined magnetic polarity and when suspended in a slightly 

 denser liquid would slowly sink and arrange themselves end on end. 

 Such a process might be helped by the crystallization of the apatite, 

 the crystals of which, forming a kind of network, would help to keep 

 the shell of films near the boundaries of the inclusion. However 

 produced, such fragile structures would be very unstable, and the 

 slightest movement in the rock would cause their collapse, malforma- 

 tion, or even obliteration. 



All of these effects can be noted in the specimens and thus add 

 support to the view, though the difficulties in the way of its acceptance 

 are so great that the suggestion must be regarded only as a crude 

 working hypothesis, which may possibly serve its purpose in the 

 elucidation of the prime difficulty — the origin of the mock-crystal 

 structure of the inclusions. 



In conclusion, the author wishes to express his cordial thanks to 

 Professor Lebour for his encouragement and interest in this work, 

 and to W, M. Hutchings, Esq., for his valuable help in the examination 

 of the rock sections. 



1 M. K. Heslop & J. A. Smythe, " The Dyke at Crookdene," etc. : Q.J.G.S., 

 vol. Ixvi, p. 1, 1910. 



