462 British Association — J. Home, F.R.8., etc. — 



sediments are separated by a line of disruption or thrust-plane from the 

 strata in the eastern part of the island. And yet, notwithstanding this 

 break, the evidence obtained in the latter district is remarkable, whatever 

 theory be adopted to explain it. There the Islay limestone and black 

 slates appear to be covered unconformably by the Islay quartzite con- 

 taining Annelid tubes and followed in ascending sequence by Fucoidal 

 shales and dolomites, suggestive of the Cambrian succession in Sutherland 

 and Ross. The Islay quartzite passes into Jura, thence to the mainland, 

 and it may eventually prove to be the Perthshire quartzite, while the 

 Islay limestone and black slate are supposed to be the prolongations of 

 the limestone and slate of the Loch Awe series in Argyllshire.^ 



From the foregoing data it will be seen that much uncertainty prevails 

 regarding the age and structural relations of the metamorphic rocks of 

 the Highlands, but the difficulties that here confront the observer are 

 common to all areas affected by regional metamorphism. 



A prominent feature in the geology of the Eastern Highlands is the 

 great development of later plutonic rocks chiefly in the form of granite 

 ranging along the Grampian chain from Aberdeenshire to Argyllshire. 

 In connection with one of these masses a remarkable paper appeared in 

 1892 which in my opinion has profoundly influenced petrological inquiry 

 in Scotland from the light which it threw on the relations of a connected 

 series of petrographical types in a plutonic complex. I refer to the paper 

 on the " Plutonic Rocks of Garabal Hill and Meall Breac," by Mr. Teall 

 and Mr. Dakyns.^ 



The authors showed that this plutonic mass comprises granite, tonalite, 

 augite-diorite, picrites, serpentine, and other compounds. Mr. Teall 

 regards the members of this sequence as products of one original magma 

 by a process of differentiation, the peridotites being the oldest rocks, 

 because the minerals of which they are composed are the first to form in 

 a plutonic magma. As the process of consolidation advances, rocks of 

 a varied composition arise, in the order of increasing acidity, viz., diorites, 

 tonalites, and granites. The most acid rock consists of quartz and 

 orthoclase, which may represent the mother liquor after the other con- 

 stituents had separated out. Mr. Teall concludes that progressive 

 consolidation of one reservoir gives rise to the formation of magmas of 

 increasing acidity, and hence that basic rocks should precede the acid 

 rocks. This theory of magmatic differentiation — so strenuously advocated 

 by Brogger, Vogt, Rosenbusch, Iddings, Teall, and others — was first applied 

 to the interpretation of varied types of plutonic masses in Scotland by 

 Mr. Teall in the paper referred to. Since then he has extended its 

 application to the granite masses in the Silurian tableland of the south 

 ■of Scotland, which include rocks ranging from hyperites at the one end 

 to granitite with microcline and aplite veins at the other. ^ Many of 

 the phenomena presented by the newer granite masses of the Eastern 

 Highlands seem to lend support to this theory. These views, indeed, 

 have permeated the petrological descriptions of the granitic protrusions 

 in the counties of Aberdeen and Argyll which have been given by Messrs. 

 Barrow, Hill, Kynaston, and Craig * in recent years. 



One of the remarkable advances in Scottish geology during the period 

 under review is the solution of the order of succession and tectonic 



' Summary of Progress Geol. Surv. 1899, p. 66. 



* Quart. Journ. Geol. Soc, vol. xlviii, p. 104. 



3 Ann. Eep. Geol. Surv., 1896, p. 40 ; see also " The Silurian Eocks of Scotland," 

 Geol. Surv. Memoir, 1899, p. 607. 



* Ann. Eep. Geol. Surv., 1897, p. 87; 1898, pp. 25-28. See also paper on 

 '^' Kentallenite and its Eelations to other Igneous Eocks in Argyllshire " : Quart. 

 Journ. Geol. Soc, vol. Ivi, p. 531. 



