Vol. 65.] THE CAULDR0N-SE.B3IDEXCE OE GLEX COE. 645 



measured at right angles to the general trend of the dykes, the 

 sum of the widths of 31 dykes amounts to about 335 yards. 

 Assuming this to be an average for the district, the amount of 

 stretching in a space of 9 miles, which is the length of the cauldron 

 measured across the direction of the dykes, is no less than 

 4683 yards, or over 2 J miles. While it can hardly be more 

 than a coincidence that the greatest diameter of the cauldron lies 

 normal to the trend of the dykes, that is, in the direction of the 

 virtual tension which admitted them, still it is evident that, at 

 the time of its formation, the cauldron was distinctly more circular 

 in form than it is now. 



We may now proceed to call attention to some important points 

 relating to the distribution of the dykes. They are most numerous 

 in the volcanic area itself, and their numbers diminish somewhat 

 rapidly both south-eastwards and north-westwards; in fact, at a 

 distance of 3 miles from the nearest outcrop of the volcanic rocks, 

 measured across the direction of the dykes, only a few isolated 

 occurrences of the latter are found. On the other hand, they pass 

 north-north-eastwards in the direction of their alignment in great 

 numbers ; they cross the River Leven 3 miles north of Glen Coe, 

 without any marked diminution, and in this direction the limit of 

 their extension has not yet been determined. On the other, or 

 south-south-western side of the cauldron, they enter the Cruachan 

 or outer ring of the Etive granite-complex, as already frequently 

 remarked, but they rapidly decrease in number as they approach 

 the margin of the central mass of the Starav Granite. 1 



If observation were restricted to this side of the Etive boss it 

 might be supposed, perhaps, that the dykes had some essential 

 connexion with the Glen Coe volcanic area, but a wider view of 

 their distribution shows that the connexion is less direct than 

 might have been supposed. It is well known that a. great series of 

 dykes, similar in composition and orientation to those of Glen Coe, 

 have been mapped by Mr. Kynaston in the southern part of the 

 Cruachan Granite ring. From the granite they stretch in a south- 

 south-westerly direction across the volcanic plateau of Lome, and 

 beyond this into Craignish and Kilmartin, a distance of nearly 

 25 miles from the margin of the Etive Granite, while a few 

 stragglers may be found even farther south still. These two 

 swarms of dykes, similar to one another in composition, trend, and 

 geological age, are thus seen to possess a distribution which is 

 symmetrical with regard to the Etive Granite. On referring to fig. 1 

 (p. 614), it will be seen that the breadth of the tract infested by 

 the Lome group of dykes is practically the same as that infested 

 by the Glen Coe group. Further, it is significant that both these 

 tracts are included between two lines which are parallel to the 



1 Later examination lias rendered this decrease doubtful. See footnote, 

 p. 641. 



