Geological Society of London. Bad 
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- 
pantellerite 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 
grains 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 Allport, 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 
reversed. 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 
crystallization and liquation going on within the magma which has 
