624 REtORT— 1901. 



part of that island there is a mass of Lewisian jjneiss overlaid unconformably hy 

 sedimentary strata which have been correlated with the lower and middle divisions 

 of the Torridon Sandstone. Unfortunately the sequence ends here, as both the 

 gneiss and overlyiog sediments are separated by a line of disruption or thrust- 

 plane from the strata in the eastern part of the island. And yet, notwith- 

 standing this break, the evidence obtained ia 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 containing 

 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 Pertbshire 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 High- 

 lands, 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 cbietly 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 1802 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 

 comple.x. I refer to the paper on the ' Plutonic Rocks of Garabal Hill and Meall 

 Breac,' by Mr. Teall and Mr. Dakyns.- 



The authors shovfed 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 constituents 

 had separated out. Mr. Teall concludes that progressive consolidation of one 

 reserpoir gives rise to the formation of magmas of increasing acidity, and hence 

 that basic rocks should precede the acid rocbs. This theory of magmatic differen- 

 tiation — so strenuously advocated by Brogger, Vogt, Rosenbuscb, 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 relations of the 

 Silurian rocks of the south of Scotland by Professor Lapworth. The history of 

 research relating to that tableland, and of all his contributions to the problems 



' Suvimari/ of Progress for 1809, p. 6G. 



- Quart Jpurn. Gcol. Soc, vol. xlviii. p. 101. 



' Atimial llcport of the Geolorjical Survey for 1896, p. 40; see also ' The Silurian 

 Rocks of Scotland,' Geological SurTey Memoir, 1899, p. 607. 



* Annual B.eport of the Geological Survey for 1897, p. 87 ; for 1898, pp. 25-28 ; see 

 also paper on ' Kentallenite and its Relations to other Igneous Rocks in Argyllshire,' 

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



