378 Geological Society : — 



vitreous material (' pitchstone ' or ' pitchstone-porphyry ') which, 

 by devitrification passes into various forms of felsite and quartz- 

 felsite. These rocks, if we class them according to the nature of 

 the porphyritic minerals they contain, fall into the several groups of 

 vitrophyric and trachytoid lavas, to which the terms pantellerite, 

 quartz-pantellerite, rhyolite, andesite, and dacite have been applied. 

 The glassy groundmass in the whole of these rocks, however, is 

 always abundant, and its characters are remarkably uniform how- 

 ever much the phenocrysts may vary. 



An admirable type of the Arran composite dykes is found at Cir 

 Mhor ; this dyke traverses the granite, and has been excellently de- 

 scribed by Ramsay and Bryce. The sides of the dyke are composed 

 of augite-andesite, having a specific gravity of 2*71 and a silica 

 percentage of 56 ; the centre of the dyke consists of a quartz-pan- 

 tellerite with glassy base (' pitchstone-porphyry ') passing into 

 various stony varieties ? this acid rock has a specific gravity of 2*36, 

 and contains from 72 to 75 per cent, of silica, it is rich in soda as 

 well as potash, and yields much water. The acid and basic rocks 

 are strikingly contrasted with one another, alike in the characters 

 of all their porphyritic crystals and of their vitreous bases. The 

 acid rocks, however, contain a few crystals which have evidently 

 been derived from the basic rock. 



There is one fact with respect to the acid rock of the Cir Mhor 

 dyke, to which attention is now called for the first time. Hyalite, 

 the clear form of opal, occurs in it as a rock-constituent, and forms 

 globules made up of several concentric layers which envelop the 

 graius of quartz, are moulded on the angles of the felspar-crystals, 

 and sometimes form independent segregations, with nuclei of tridy- 

 mite-crystals. The identity of this interesting mineral has been 

 established, not only by its optical properties, but by various micro - 

 chemical tests. 



On the shore and in the cliffs at Tormore, on the west coast of 

 Arran, there occurs a remarkable plexus of dykes, many of which 

 supply striking illustrations of our second class of ' composite dykes.' 

 The locality was first described by Jameson, and some of the dykes 

 have since been noticed by Airport, Zirkel, and other authors. In 

 these composite dykes both the basic and the acid rocks are present ; 

 but while in some cases the acid rock was clearly introduced after the 

 basic, in others the order of ejection of the two materials was re- 

 versed. In some instances, the plane of weakness along which the 

 opening and re-injection of the dyke took place follows its central 

 line ; at other times it is at the side of the dyke ; and occasionally it 

 traverses the dyke-mass in a sinuous manner. The younger rock 

 often contains numerous crystals derived from the older one, and 

 these often show much alteration from the action of the magma in 

 which they have been caught up. It is evident that the interval 

 between the first and second injection of these dykes was sufficiently 

 long to allow of the complete consolidation of the older rock. 



In conclusion, it is pointed out that, while the peculiarities of 

 the first class of composite dykes can be accounted for by selective 



