Yol. 52.] GRANOP.EYRES OF STRATH (SKYE). 323 



Carrock Eell granophyre near its contact with a highly basic gabbro 

 have been described by the present writer, 1 but there the disso- 

 lution of the derived material has been much more complete, and 

 the analogy with the present case is more remote. 



The xenoliths 2 in our granophyres are, as a rule, less than an 

 inch in diameter, and have ill-defined outlines. Those readily 

 recognized and identified by the eye as distinct foreign fragments 

 are not common. In the thin slices undestroyed xenoliths are not 

 frequent (6704), but altered xenocrysts are universally found. 



Most of this derived material has undoubtedly come from a 

 gabbro, and from one closely comparable with the ordinary gabbros 

 of the district, such as that which occupies a considerable tract to 

 the north-east of the Red Hills. Of these gabbros, as seen in situ, 

 a summary description will suffice. They consist essentially of 

 felspar and augite. The felspar is usually a labradorite, often in 

 idiomorphic crystals showing some zonary banding between crossed 

 nicols. The augite is pale brown to almost colourless in thin slices. 

 Instead of the true diallage-structure, parallel to the orthopinacoid,. 

 it has usually a delicate striation 3 parallel to the basal plane, often 

 emphasized by a more or less pronounced schiller-structure. This 

 frequently affects only part of a crystal, and it imparts a deeper 

 brown tint to the slice. A rhombic pyroxene is rarely met with in 

 the gabbros of this district, though it occurs in some of the coarser 

 rocks of the Cuillin Hills, farther west. Recognizable olivine is 

 not common, although it may sometimes be concealed by secondary 

 magnetite-dust, as remarked by Prof. Judd. Original magnetite 

 often occurs, in shapeless grains or patches. Needles of apatite 

 are met with, but by no means constantly. 



The gabbro-debris in the granophyre is seen in the thin slices in 

 different stages of dissolution, but is for the most part completely 

 disintegrated by the caustic or solvent action of the acid magma on 

 some of its minerals. Those constituents which resisted such action 

 have been set free, and now figure as xenocrysts, either intact or 

 more or less perfectly transformed into other substances. At the 

 same time the material absorbed has modified the composition of 

 the magma, in the general sense of rendering it less acid, and this 

 is of course expressed in the products of the final consolidation of 

 the granophyre. In order to present in systematic form the observ- 

 ations made, it will be convenient to begin by enquiring what has 

 befallen each of the chief constituents of the gabbro. 



1 Quart. Journ. Geol. Soc. vol. li. (1895) pp. 125-139. 



2 [For convenience, I adopt Prof. Sollas's terms, ' xenolith ' for an enclosed 

 foreign rock-fragment, and ' xenocryst ' for an isolated crystal of foreign 

 derivation.] 



3 [I follow Mr. Teall (Quart. Journ. Geol. Soc. vol. xl. 1884, pp. 646, 647) 

 in terming this structure simply a ' striation.' It does not appear to be a twin- 

 lamellation, and I have not been able to satisfy myself as to its true nature. 

 Mr. Teall regarded it, in the Whin Sill, as of secondary origin ; but I have 

 seen nothing leading to this conclusion in the gabbros of Skye, of Carrock Fell, 

 of St. David's, etc., and its occurrence in ' xenocrysts ' as detailed b'4ow would 

 be difficult to reconcile with such a supposition. —March 12th, 1896.] 



Q. J. G. S. No. 206. z 



