306 G. W. Tyrrell— Tertiary Dykes 



The dykes of Arran have been the subject of geological inquiry 

 since the beginning of the nineteenth century (Harker, 1903, 

 Bibliography, pp. 181-90 l ). Their extraordinary abundance has 

 attracted much attention, but little is as yet known of their petro- 

 graphical characters. Still more is this the case in regard to the 

 Tertiary dykes of Bute, the Cumbraes, and the Ayrshire and 

 Argyllshire mainland. Sir A. Geikie has gathered together all that 

 was previously known of the geological and petrographical characters 

 of the Tertiary dykes, and has supplemented it with the facts gained 

 during his own numerous traverses of the Tertiary volcanic districts 

 (1897, pp. 118-80). He distinguishes the great solitary dykes 

 (e.g. the Cleveland dyke of the North of England) from the gregarious 

 dykes, which are shorter, narrower, more basic, than the solitary 

 dykes, and are closely crowded together in restricted areas. 

 Petrographically he divides them into four groups — (1) normal 

 basalts and dolerites; (2) andesites ; (3) trachytes (Cowal) ; (4) acid 

 dykes, felsite, quartz-porphyry, pitchstone, etc., which are arranged 

 in the order of age. The first group, basalts and dolerites, include 

 by far the greater number of the dykes, especially those of the 

 gregarious type, whilst the great solitary dykes generally belong to 

 the second group, the andesites. 



The Geological Survey memoir on Cowal (1897, pp. 126-71) 

 contains abundant geological information as to the Tertiary dykes of 

 that part of the Argyllshire mainland bordering the Firth of Clyde. 

 Sir J. J. H. Teall's petrographical descriptions of some of these 

 dykes suggest relationships to the Cumbrae dykes described later in 

 this paper. 



The Tertiary igneous rocks of Arran, South Bute, and the 

 Cumbraes are the subject of a chapter by Dr. A. Harker (1903, 

 pp. 103-27) in the Geological Survey memoir on that area. In this 

 chapter the Tertiary dykes are briefly treated under the headings 

 dolerite and basalt, augite-andesite, pitchstone, and devitrified 

 pitchstones. One or two Arran dykes with special characters have 

 been previously described by me (Tyrrell, 1913, 1916) ; and Dr. "W. R. 

 Smellie (1916) has recently described some of the Tertiary dykes 

 of Bute. 



The Cumbrae Type of Tertiary Dyke (Cumbraite). 



The Great Cumbrae is traversed by several large and finely exposed 

 Tertiary dykes of a peculiar petrographical character and striking 

 macroscopic appearance. They consist of a black glossy rock, with 

 a tendency to vitreous lustre, in which are embedded numerous 

 phenocrysts of white or yellow, fresh, plagioclase felspar, which 

 proves to be near anorthite in composition. In the field they form 

 prominent wall-like exposures trending to the N.N.W., upon the 

 lowest raised beach which encircles the island. The Lion Ilock, 

 near Keppel Pier, is one of these dykes which has been broken in 

 such a way that its profile suggests a couchant lion. It rises to 

 a height of 15 feet, with a thickness of 16 feet, whilst a second dyke 



1 See list of references at end of paper. 



