G. W. Tyrrell — Intrmiom of Kilsyth-Croy Didrid. 363 



essentially the same as the rocks described under various names by 

 Elsden from the St. Davids area,^ by Harker from Carrock Fell,- by 

 SoUas from Barnavave, Carlingford,^ and by Falconer from the Bath- 

 gate Hills/ If the search for similar or identical rocks be carried 

 beyond these islands, it is found that diabase with micropegmatite 

 has a worldwide distribution. In Europe it has been described by 

 Tornebohm from Sweden and by Barrois from Brittany ; in Asia it 

 has been described by Holland from South India ; in North America 

 it occurs in the Lake Superior region and in New England, as described 

 by Lawson, Lane, Hawes, and others ; in South America Harrison has 

 described it as occurring in great abundance in the interior of British 

 Guiana ; in Africa Howe has described it from the north-eastern 

 territories of the Congo Free State ; and lastly it has been described 

 by Prior as occurring at South Victoria Land in the Antarctic 

 Continent. 



A feature of these rocks is the remarkably constant chemical com- 

 position of the normal type, containing plagioclase, pyroxenes, micro- 

 pegmatite, and iron-ores in proportions similar to those of Table II. 

 This type makes up the main mass of the intrusions, and must be 

 regarded as representing the actual mass composition of the magma 

 from which the rock was immediately derived. The sometimes 

 highly basic marginal facies and the granophyric veins must be 

 regarded as opposite differentiation products of this magma. Table III 

 gives the chemical composition of a few selected foreign diabases 

 with micropegmatite. These should be compared with the British 

 examples given in Table I. 



The mode of occurrence of this rock is also distinctive. It always 

 occurs in thick, massive, vertical - sided dykes, which sometimes 

 continue for many score miles across country, and also as thick, 

 laccolitic protrusions from such dykes. The great E.-W. dykes of 

 the Midland Valley of Scotland are a classic example. The great 

 dykes described by Lawson in the Rainy Lake district of Canada 

 are of exactly the same type.* They are all more or less vertical, 

 traverse all formations indifferently, continue for great distances (in 

 one case 100 miles), and are of fairly uniform width (60-150 feet). 

 They are diabases with micropegmatite, precisely similar to the 

 Scottish examples. Moreover, the fissures along which they occur, as 

 in Scotland, are not necessarily faults. The St. Davids Head rocks, 

 according to Elsden,^ occur in thick vertical sills between almost 

 perpendicular Arenig shales. The augite-diorites with micropegmatite, 

 described by Holland from South India, occur as a great series of basic 

 dykes piercing the older crystallines of that area. One of them is 

 100 yards wide. An enormous number of these dykes, with their 

 associated laccolites, occur in the interior of British Guiana. They 

 pierce a basement of Archaean gneisses, granites, and schists, and are 



1 Op. cit., p. 28. 2 Q.J.G.S., 1894, vol. i, p. 311. 



' Trans. Royal Irish Acad., 1894, vol. xxx, pp. 477-512. 



* Op. cit., p. 28. 



* " Report on Geol. of Rainy Lake Region" : Ann. Rep. Geol. Suivey Canada, 

 1887-8, pt. i, pp. 147-50. 



6 Q.J.G.S., 1908, vol. Ixiv, p. 273. 



