440 MR. J. PARKINSON ON AN INTRtTSIGN OF GRANITE [Aug. 1 899, 



clear, external zone, is for the most part, at all events, plagioclase. 

 There is also a certain amount of iron-oxide. 



A slide prepared from a specimen of an incompletely dissolved 

 fragment which lay in the granitic intermediate rock described 

 on p. 435, shows a much more regular distribution of its con- 

 stituents than the last. The permeation by the acid magma has 

 been much more complete than in the preceding rock. It is 

 traversed by one or two quartz -felspar veins, but otherwise is an 

 uniform, rather fine-grained rock showing about equal proi)ortions 

 of a brown-red felspar (together with quartz) and black minerals 

 — hornblende and mica. Large typical irregular orthoclases, and a 

 few rather smaller crystals of plagioclase, lie in a granular mosaic of 

 quartz and felspar, with hornblende- and mica-crystals of the usual 

 types : the former occasionally of some size, but as a whole the 

 latter predominate. A most intimate relation exists between these 

 two constituents ; flakes of the biotite are embedded in the green 

 hornblende in a manner which suggests strongly the production of 

 the former from the latter. Although the orthoclase-crystals pre- 

 dominate in one part of the slide, yet there is no definite separation 

 into a more acid and a more basic region. Plagioclase is common, 

 and often surrounded by a layer of unstriped felspar giving a 

 practically straight extinction. In a few cases these external 

 zones show striping, in continuation of the central older cores. 

 The latter are corroded, and often have almost a granular ap- 

 pearance, so completely do the edges and corners appear to have 

 been removed. It is difficult to make a positive statement, but 

 it is most probable that these cores represent the remnants of the 

 labradorite of the diabase. 



A peculiar, and at the same time not uncommon, type of rock 

 is distinguished by containing a multitude of white or pink ortho- 

 elase-crystals rather irregular in shape, as though corroded, and 

 of large size, up to about -45 inch in length, set in a fine-grained dark 

 matrix. The rock occurs at the junction of granite and diabase, 

 and here and there the disposition of the felspars indicates movement 

 prior to consolidation. A thin section shows less quartz than in the 

 allied rocks above described. It bears a very close resemblance to 

 tnat described by Prof. Cole, from Glasdrumman Port, Co. Down.^ 



The spotted structure, well shown in the orthoclases, has been 

 already considered. The dominant mineral in the matrix is the 

 usual green hornblende : this is scarcely ever idiom.orphic. Here and 

 there a lighter, rather fibrous centre, together with opaque and 

 reddish dust, probably represents the remains of an original ;mgite. 

 Biotite occurs sparingly ; the quartz fills up interstices between 

 the other constituents, usually showing optical continuity over a 

 considerable area. Occasionally a corroded basic felspar is seen. 



y. Junction between the Granite and the Diabase. 



We may now consider the petrographical features presented by 

 the two rocks at a distinct junction. The veins of granite which 

 1 Sci. Trans. Eoy. Dub. See. ser. 2, vol. v (1894) p. 239. 



