414 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [June 16, 



In his Geological Map of Scotland Dr. MaccuUoch lays down a 

 band of igneous rock as extending continuously from the bay east 

 of Campbeltown to the west coast between Losset and Machrihanish. 

 This band has no existence in reality, the porphyry quarried to the 

 east of the town being soon interrupted by the slate and limestones, 

 and igneous rocks only again appear near Kilkivan, about four miles 

 distant. On the other hand, a large portion of the southern district, 

 coloured by him as mica-slate, is a nearly continuous mass of igneous 

 rocks, intermixed with or overlying secondary strata. In the district 

 north of Campbeltown Loch a large outburst of igneous rocks has 

 also taken place, of which no trace appears in his map. The most 

 abundant igneous rocks are varieties of felspar and claystone por- 

 phyries, and then several kinds of greenstone. Of the former the 

 porphyries of Davar island, formerly described by Dr. MaccuUoch, 

 are the most remarkable, and fully merit the praise he has bestowed 

 on them. The most common have a clove-brown felspar basis, with 

 imbedded crystals of labradorite. In others the basis shows purplish 

 tints with disseminated light green crystals, and others again have a 

 green basis with clove-brown crystals. The stone is always crystal- 

 line and of a very firm, uniform texture, and seems admirably fitted 

 for ornamental works, whilst the island would furnish an inexhaust- 

 ible supply with the most ready means of transport. This mass of 

 porphyry rises up in an isolated position from the sea, and there is 

 thus no means of determining its age ; but many of the boulders in 

 the red conglomerate closely resemble this rock, and, if derived from 

 it, would carry its origin back to a very remote epoch. 



The porphyries on the mainland are generally far less hard and 

 crystalline than those of Davar island. The chief exception is a mass 

 of porphyry or syenite seen on the road to the Light-house in the 

 slates of the Mull district. This is a very hard rock with a basis of 

 flesh-red orthoclase, in which are disseminated a considerable amount 

 of a greenish white mineral, perhaps oligoclase, but much decom- 

 posed in the specimens I brought away, then dark green hornblende 

 in minute crystals, and grains of quartz. The other porphyries are 

 often claystones or compact felsites, like those of the Braid Hills or 

 Pentlands near Edinburgh, and some of them even mere granular 

 tufas, almost like sandstones. 



As these rocks intrude on the coal-formation, they cannot be older 

 than its deposition, and not improbably are much newer. One of 

 them, of a reddish yellow colour, quarried near Kilkivan, contains 

 angular fragments of shale converted into a kind of jasper, but still 

 retaining all the laminar texture of the original rock. One of these 

 altered fragments consisted of almost innumerable layers of shale, not 

 much thicker than paper, but readily distinguished by their differ- 

 ences of colour or tint. 



The other igneous rocks are augitic traps, partly greenstones or 

 dolerites, partly compact or basalts. One of the most remarkable of 

 them is a basaltic breccia which forms the summit of Knock-Scalbert, 

 immediately north of Campbeltown. It consists entirely of angular 

 fragments, generally not larger than a common marble, but all loosely 



