of the Clyde Area. 355 



concerning the thousands of Tertiary dykes in the Clyde area it is 

 already clear that they show considerable petrographic diversity, and 

 that the various groups have differing relations in regard to their 

 size, length, direction, age, and degree of connexion with the local 

 centres of Tertiary volcanic activity. Only rocks of andesitic or 

 basaltic composition have been dealt with in this paper. There is 

 a well-marked division into two groups : («) a tholeiitic group, 

 characterized by the presence of glass, intersertal texture, phenocrysts 

 of basic felspar, and presence of occult quartz ; and (b) a crinanite- 

 olivine-dolerite group, with coarse ophitic texture, with interstitial 

 analcite or radial zeolites, rich in olivine, and free from glass or 

 phenocrysts. The tholeiitic group ranges in chemical composition 

 from sub-acid types (Anderson & Radley, 1916, p. 210) to 

 thoroughly basic types such as those of Corrie and Largs. The 

 crinanite-olivine-dolerite group, however, shows no significant 

 variation, and remains uniformly basic. It is probable that other 

 groups will be found as investigation proceeds, especially in Arran, 

 where only a tithe of the dykes (and related sills) have been closely 

 examined. It will be necessary in future to define the relation, if 

 any, between the great pitchstone-felsite group of Arran and the 

 tholeiitic series. There may also be a relation between the crinanites 

 and the basalt-felsite composite sills and dykes of the south of Arran. 

 I must acknowledge with gratitude the assistance received, in this 

 work from Dr. H. H. Thomas, Petrographer to the Geological Survey, 

 who has not only loaned to me all the Survey slides bearing on the 

 subject, but has looked through many of my slides and compared the 

 rocks with the types occurring in Mull. I am also much indebted 

 to Mr. Chetai Yu and Dr. Alex. Scott for the excellent chemical 

 analysis of cumbraite in this paper. 



Bkfkrences. 



Anderson (E. M.) and Eadley (E. G.). 1916. "The Pitchstones of Mull 



and their Genesis " : Q.J.G.S., Ixxi, pp. 205-17. 

 Clough (C. T.). 1897. In The Geology of Cowal, Mem. Geol. Surv., 



pp. 126-71. 

 Flett (J. S.). 1911. In The Geology of Knapdale, Jura, and North Kintyre, 



Mem. Geol. Surv., pp. 116-18. 

 Geikie (Sir A.). 1880. " The ' Pitchstone ' of Eskdale " : Proc. Roy. Phys. 



Soc. Edinb., vol. v, pp. 219-55. 



1897. Ancient Volcanoes of Great Britain, vol. ii, pp. 118-80. 



Geological Survey. 1914. Summary of Progress for 1913. 



1915. Summary of Progress for 1914. 



Harker, A. 1903. In The Geology of North Arran, South Bute, and the 

 Cumbraes, Mem. Geol. Surv., chapter on the Petrography of the Tertiary 

 Igneous Eocks, pp. 103-27. 



1908. Petrology for Students, 4th ed. (Brunton dyke). 



Heslop (M. K.) and Smythe (J. A.). 1910. " On the Dyke at Crookdene 



(Northumberland) and its Relations to the Colly well, Tynemouth, and 



Morpeth Dykes" : Q.J.G.S., vol. Ixvi, pp. 1-18. 

 Smellie (W. R.). 1916. " The Igneous Rocks of Bute " : Trans. Geol. Soc. 



Glasgow, vol. xv, pt. hi, pp. 334-73. 

 Teall (J. J. H.). 1884. " Petrological Notes on some North of England 



Dykes " : Q.J.G.S., vol. xl, pp. 209-47. 



British Petrography, pp. 194-7, 200-7, pis. xii, xiv, xxiv, 



1, XXX. 



