THE CLYDE TERRITORY. 411 



andesite, and others of diabase, basalt, or dolerite. The intrusive sheets of 

 that epoch likewise consist mainly of andesite or trachyte. Mr. Gunn has 

 observed that in Bute and the Cumbraes the dykes of trachyte, basalt and 

 dolerite sometimes pass into intrusive sheets. A prominent series of sills of 

 a more basic character is largely developed along the northern belt of the 

 Carboniferous Limestone series in the midland valley. These dolerite sills 

 occur at Kilbarchan, Johnstone, and Paisley, and they extend at intervals 

 from Milngavie by Kilsyth to Stirling, where one of these intrusive sheets 

 forms a conspicuous feature in the Castle Eock. These basic sills are linked 

 with the puy-eruptions of Carboniferous Limestone time by Sir A. Geikie. 



In the Ayrshire basin of Carboniferous strata there is an oval-shaped area 

 of volcanic and sedimentary rocks which have been referred to the Permian 

 period. They rest on the upper or red sandstone group of the Coal-measures. 

 The margin of this Permian basin consists of sheets of lava and volcanic 

 tuffs, interstratified with brick-red sandstones, and these are overlaid by 

 false-bedded red sandstones. The lavas are well seen in the course of the 

 river Ayr above Ballochmyle, and the red sandstones in the large quarries 

 near Mauchline. Outside the Permian basin, numerous necks or vents, 

 filled with volcanic agglomerate, pierce the Coal-measures which have 

 been referred to this period of volcanic activity. Since the discovery of 

 Khaetic and Liassic fossils in Arran, which has led to the grouping of the 

 red sandstones in the south of Arran with the Triassic strata, it has been 

 suggested that the Mauchline sandstones may be of the same age. 



The part of Arran lying to the south of a line drawn from Brodick to 

 Blackwaterfoot on the west coast is occupied by a great development of 

 false-bedded red sandstones, conglomerates and marls, which are now regarded 

 as of Triassic age. This correlation is based on the discovery of Rhaetic 

 fossils characteristic of the Avicula contorta-zone, and Lower Liassic fossils 

 in masses of shale, marl, limestone, and sandstones, which do not now occur 

 in place on the island. These portions of sedimentary strata are associated 

 with fragmental volcanic materials, and various intrusive igneous rocks 

 which now fill a great volcanic vent. Its site is about halfway between 

 Brodick Bay on the east and Machrie Bay on the west, and to the south of 

 the String Road which crosses the island from Brodick to Shiskine. This 

 discovery was made by Messrs. Peach, Gunn, and Macconochie, in the course 

 of the Geological Survey of that district. The occurrence of these sedi- 

 mentary masses, one of which is acres in extent, points to the former existence 

 in the Clyde area of Ehaetic and Lower Liassic strata which have now been 

 swept away. The detailed mapping of the island by Mr. Gunn has con- 

 clusively proved that the Triassic sandstones rest partly on the Coal-measures, 

 partly on the Carboniferous Limestone series, and partly on the Old Bed 

 Sandstone. Hence the apparent conformability is entirely deceptive. A 

 patch of these rocks occurs in the north of the island at the Cock of Arran 

 where the late Mr. James Thomson many years ago discovered pebbles with 

 Carboniferous Limestone fossils in the Triassic conglomerate, though at the 

 time he referred the conglomerate to the horizon of the Millstone Grit. 



In addition to the Rhaetic and Liassic strata in the volcanic vent near 

 Shiskine, fragments of limestone and chert were found associated with the 

 agglomerate by the Geological Survey. The limestone has yielded fossils of 

 Cretaceous age, including Globigerina, Tertularict, Porosphaera globularis, 

 and Inoceramus, thus pointing to the existence of Cretaceous strata in the 

 Clyde area, which have been removed by denudation. It is obvious, there- 

 fore, that this vent is of post-Cretaceous age, and may with probability be 

 referred to the age of the necks of Tertiary time in Skye. This discovery 

 likewise furnishes valuable evidence in favour of referring the granite mass 

 in the north of Arran, and most of the intrusive, acid and basic igneous 

 rocks in that island to the Tertiary period. 



