Vol. 51.] GRANOPHYRE AND THE GRAINSGILL GREISEN. 139 



Mr. Bayley has kindly sent me a few specimens of the Pigeon 

 Point rocks lor comparison with my own. So far as can be judged 

 from these, the relations may well be the same in the two districts. 

 The red rock is a granophyre differing from that of Carrock Pell in 

 having biotite instead of augite, and in containing idiomorphic 

 crystals of quartz as well as of plagioclase. In one slide the micro- 

 graphic intergrowth which makes up the bulk of the rock is on a 

 very minute scale, and passes into what I have styled the crypto- 

 graphic structure. The ' intermediate ' rocks, as in the Carrock Pell 

 district, are somewhat coarse in texture. They are granophyres, in 

 that they have a groundmass of felspar and quartz with micrographic 

 arrangement, but in other respects they differ from the ' red rock.' 

 Iron ores and apatite have come in rather abundantly, and biotite 

 is now only subordinate, the dominant ferro-magnesian mineral in 

 the slides examined being an augite with basal lamellation and 

 ' herring-bone ' structure. 



[Prof. Sollas has worked out very thoroughly the modification of 

 a granophyre-magma by the absorption of basic material in the case 

 of the numerous dykes which traverse the gabbro of Barnavave, 

 Carlingford. 1 The action of the acid magma npon the derived 

 crystals is much more clearly exhibited there than in the rocks 

 which I have studied. The relations between the earlier gabbro 

 and the later granophyre are evidently different in the two areas. 

 At Barnavave the gabbro appears to have been traversed by con- 

 traction-joints and fractured by earth-movements before the injection 

 of the granophyre ; and, in consequence of this and of a miarolitic 

 structure in the gabbro, the acid magma was able to penetrate the 

 basic rock in an extraordinarily intimate manner. Prof. Sollas 

 gives reasons for believing that the ' granophyric gabbro ' of Barna- 

 vave has been produced from ordinary gabbro by this process of 

 impregnation, and suggests that the same explanation may apply to 

 Carrock Fell. According to my observations, however, such action 

 does not there extend to more than a foot or two from the well- 

 defined line of junction with the main body of granophyre, while 

 the quartz-gabbro constituting the central part of the basic intrusion 

 gives evidence of an entirely different origin. — March 2nd, 1895.] 



Part III. The Grainsgill Greisen. 

 13. Description of the Greisen and its Relation to the Granite. 



The quartz-mica rock which is here styled 'greisen'is, as already 

 intimated, a peculiar modification of the Skiddaw granite, and the 

 simple course in describing it will be to treat it at once from this 

 point of view. 



The well-known Skiddaw granite is seen in three isolated but 

 neighbouring exposures, each at the bottom of a valley and with 

 the appearance of underlying the surrounding slates with a quasi- 



1 ' On the Volcanic District of Carlingford and Slieve Gullion : — Part I. 

 On the Belation of the Granite to the Gabbro of Barnavave, Carlingford,' 

 Traus. Roy. Irish Acad. vol. xxs. (1894) pp. 477-512. 



