252 Dr. J. A. Smythe — Inclusions in 



contain vesicles and kernels of tlioronghly acidic composition, and 

 are associated in the same area with dykes of acidic pitchstoue.' 



The Origin of the Liclusmis. 



The observations described in the foregoing pages throw some light 

 on the origin of the inclusions, though many of the deductions are 

 necessarilv of a somewhat speculative nature, and some of the 

 difficulties are probably incapable of solution by the data at present 

 available. The three main aspects of the problem are : how was the 

 material of the inclusions generated, how did it get into its present 

 position, and by what process did the inclusions acquire their form 

 and charactei'istic facetting? 



"With regard to tl)e first point, there is nothing to indicate that the 

 inclusions represent foreign material, igneous or sedimentary, which 

 has been caught up by the whin during intrusion; and the evidence, 

 both chemical and petrological, makes it clear that local variation of 

 the whin as a whole is not at the root of the matter. In the lack 

 of definition, or of sharp contact between the inclusions and the 

 surrounding rock, and in the absence of tangentially-arranged 

 felspars around them, the inclusions differ from the glass-filled 

 amygdules of many of the local dykes, ^ and thus their material 

 cannot be regarded as a differentiation -product of the whin, produced 

 in situ by the process of crystallization, and concentrated locally by 

 injection into amj-gdaloidal cavities. 



That the material has been derived from the whin, however, by 

 some process of fractionation seems evident from its associations^ 

 its likeness in composition to the felspars, which are the early 

 crystallization-products from that magma, and especially by the 

 chemical relationsliip it bears to tliat rock. 



In view of these considerations and also of the field-relationships, 

 the lower specific gravity of the inclusions, and the very frequent 

 occurrence in them of amygdaloidal cavities, it seems inevitable to 

 conclude that the material of the inclusions is derived from the whin 

 and has floated upwards into its present position from its source below. 



Now at this place the whin forms a comparatively thin sill, so 

 that one must further assume the sill to be connected with a deep- 

 seated source of supply. The suggestion is thus made that the Whin 

 Sill at Snook Point is directly connected with a feeder of some sort; 

 that the sill itself is composite in the sense that several successive 

 injections of magma have taken place; and that after the final 

 injection, and before its consolidation, small masses of magma, 

 specifically lighter, of different composition, and possibly buoyed 

 np by gus-bubbles, floated up the feeder and became entrapped in 

 the viscous basalt of the last-injected sheet, in the immediate 

 neighbourhood of the feeder, and in the feeder itself. 



This liypothesis accounts s;ttisfactorily for many peculiarities in 

 the mode of occurrence of the inclusions, as, for example, their 

 restricted range to the dyke-like portion of the rock and the sheet 



■* A. Harker, The Tertiary Igneous RocTis of Shye, 1904, p. 402. 

 2 J. J. H. Teall, Geol. Mag., Dec. Ill, Vol. VI, p. 481, 1899. M. K. 

 Heslop & J. A. Smythe, Q.J.G.S., vol. Ixvi, p. 1, 1910. 



