of the Clyde Area. 



353 



minor constituents of the groundrnass. The interstices are filled by 

 a colourless glass, darkened by innumerable globulites. Quantitatively 

 the glass forms probably less than 10 per cent of the total rock. 

 Minute spherical vesicles filled with dark glass are rather abundant. 1 



Bocks of this type occur freely near Corrie, but have not yet been 

 found elsewhere. They vary within small limits of quantitative 

 relations and texture. 



The second type may be known as the Largs type owing to the 

 occurrence of two beautifully fresh dykes on the shore near the town 

 of Largs, Ayrshire. An extended description of these rocks is 

 unnecessary, as they differ from the Corrie type only in the much 

 greater abundance of olivine, not only as granular constituents of the 

 groundrnass, but as large phenocrysts. In the groundrnass (Fig. 3a) 

 there is an augmented proportion of augite relative to the other 

 constituents, and a diminished, almost vanishing, proportion of 

 glassy base. A dyke of this character also occurs in the gorge known 

 as Creag Bhan, Whiting Bay, Arran. This type may be regarded as 

 the most basic of the tholeiitic series of Tertiary dykes. 



Fig. 3a. — Olivine-tholeiite (Largs type), N.N.W. dyke, shore £ mile south of 

 Pier, Largs, Ayrshire. x 20. Phenocrysts of anorthite and olivine ; 

 groundrnass : laths of labradorite, grains of augite and olivine, in scanty 

 base of dark glass, x 20. 

 ,, 36. — Crinanite, N.N.E. dyke, high road above Cordon, Lamlash, Arran. 

 x 20. Ophitic plexus of titanaugite and labradorite laths, with 

 abundant olivine and magnetite. Fresh, colourless analcite in triangular 

 interstices between the felspars. 



Ophitic Olivine-Dolerite and Crinanite (Fig. 3b). — The dykes 

 of this group stand apart from those belonging to the tholeiite series, 

 and, as far as present knowledge goes, appear to belong to an earlier 

 phase of Tertiary volcanic activity in the Clyde area. A crinanite 

 dyke from Whiting Bay, Arran, was described some years ago 

 (Tyrrell, 1913); but a large number of these dykes have since been 



1 For a figure of this rock see Tyrrell, 1909, PI. XIX, Fig. 5. 

 decade vi. — VOL. IV. — NO. VIII. 23 



