22 MB. E. H. CUNNING HAM-CRAIG OX [Feb. 1904, 



(d) The Green Beds. 



The progressive metamorphism of the Green Beds cannot be 

 traced in this area in detail, as there are no exposures of these 

 strata between Beinn Lomond and Ardlui. It is not necessary to 

 describe now the alteration that they have undergone, a description 

 which I hope to give in a future communication. It will be sufficient 

 to state here that the metamorphism of these well-known rocks 

 bears out the conclusions as to progressive metamorphism arrived 

 at after examination of the rocks of the Beinn-Ledi Group. On 

 the southern slopes of Beinn Lomond they are epidotic and chloritic 

 grits ; at .Ardlui they are hornblende-schists, net easy to distinguish 

 from hornblende-schists of igneous origin. 



• VI. Chemical Analyses. 



The stratigraphicai relations of the albite-gneisses having been 

 proved by a study of the structure of the district, and by the identi- 

 fication of the Green Beds at Ardlui, we are impelled to the conclusion 

 that they have been formed by the action of dynamic and construc- 

 tive metamorphism from the Beinn-Ledi Grits, unless we are to 

 assume a change in lithological character and chemical composition 

 in the rocks of the Beinn-Ledi Group when traced north-westward. 

 Chemical analysis was necessary to prove whether or not such a 

 change existed. I was inclined to disbelieve in such a change, 

 but I found, after arriving at the conclusion stated above, that 

 Continental geologists favoured the view that albite-gneisses of 

 similar character had been formed by the metamorphism of phyllites 

 rather than grits. 



Mr. Teal], in the Appendix to the Survey Memoir on the ' Geology 

 of Cowal' (1897) p. 297, refers to the occurrence of rocks con- 

 taining authigenic albite in the northern border of the central zone 

 of the Eastern Alps. These rocks were described by A. Bcehm, 1 

 who denned the type as transitional between the old crystalline 

 schists and the true phyllites. Mr. Teall also refers to the albite- 

 phyllites of Saxony," and of the Green Mountains of Massachusetts. 3 

 In all these instances the association of minerals appears to be 

 similar : white mica, chlorite, and folia of quartz accompanying the 

 albite. 



The suggestion is made in some, if not all, of these cases that the 

 albite-gneisses have been formed from phyllites, but the descriptions, 

 especially in the case of the Green Mountains, hardly seem to uphold 

 this idea. Prof. Wolff, in his description of these schists or gneisses 



1 ' Ueber die Gesteine des Wechsels ' Tschermak's Min. u. Petr. Mitth. n. s. 

 vol. v (1883) p. 197. 



2 K. Dalmer, ' Erlaiiterungen zur geologischen Specialkarte des Konigreichs 

 Sachsen — Section Lossnitz' 1881. 



3 Monogr. U.S. G-eol. Surv. vol. xxiii (1894). 



