Presidential Address to Geological Section. 461 



gneiss covering many square miles of ground and presenting many of the 

 structures so characteristic of that complex in the undisturbed areas 

 already described. Within the belt of Lewisian gneiss at Glenelg 

 Mr. Clough has mapped a series of rocks presumably of sedimentary 

 origin, including graphitic schists, mica schists, and limestones, but the 

 gneiss with which they are associated possesses granulitic structure like 

 that of the adjoining Moine schists.^ Further, in the east of Sutherland, 

 and also in the county of Ross, foliated and massive granites appear 

 which are interleaved in the adjoining Moine schists, forming injection 

 gneisses and producing contact metamorphism.^ 



In the Eastern Highlands the Moine series disappears and is replaced 

 by a broad development of schists, admittedly of sedimentary origin, 

 which have been termed the Dalradian series by Sir A. Geikie. Within 

 recent years it has been divided into certain rock-groups which have been 

 traced by the Geological Survey from the counties of Banff and Aberdeen 

 to Kintyre. It has been found that, though highly crystalline in certain 

 areas, they pass along the strike into comparatively unaltered sediments, 

 as proved by Mr. Hill in the neighbourhood of Loch Awe.-* Before the 

 planes of schistosity were developed in these Dalradian schists they were 

 pierced by sills of basic rock (gabbro and epidiorite) and acid material 

 (granite), both of which must have shared in the movements that affected 

 the schists, as they merge respectively into hornblende schists and foliated 

 granite or biotite gneiss. Both seem to have developed contact meta- 

 morphism ; indeed, Mr. Barrow * contends that the regional metamorphism 

 so prominent in the South-East Highlands is mainly, if not wholly, due 

 to the intrusion of an early granite magma, now exposed at the surface 

 in the form of local bosses of granite and isolated veins of pegmatite. 



The age of the Dalradian schists has not been determined. Though 

 there seems to be an apparent order of superposition, in this series it is 

 still uncertain whether that implies the original sequence of deposition. 

 Since Sir A. Geikie applied the term Dalradian to the Eastern Highland 

 schists in 1891,^ evidence has been obtained ^ that suggests the correlation 

 of certain rocks along the Highland border with the Arenig and younger 

 Silurian strata of the Southern Uplands. Consisting of epidiorite, chlorite 

 schist, radiolarian cherts, black shales, grits, and limestone, they have 

 been traced at intervals from Arran to Kincardineshire. In the latter 

 region Mr. Barrow contends that they are separated by a line of dis- 

 ruption from the Highland schists to the north ; but no such discordance 

 has been detected in the Callander district or in Arran. Though these 

 rocks of the Highland border have been much deformed, yet their occurrence 

 in the same order of succession in that region and in the Southern Uplands 

 is presumptive evidence for their correlation. 



In view of this evidence it is not improbable that the Dalradian series 

 may contain rock-groups belonging to different geological systems. Indeed, 

 the result of recent Survey work in Islay tends to support this view. 

 For in the south-west part of that island there is a mass of Lewisian 

 gneiss overlaid unconformably by 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 overlying 



1 Summary of Progress Geol. Surv. 1897, p. 37. 



2 " On Foliated Granites and their Relations to the CrystalHne Schists in Eastern 

 Sutherland" : Quart. Journ. Geol. Soc., vol. lii, p. 633. 



3 Ann. Rep. Geol. Surv., 1893, p. 265. 



* "Intrusion of Muscovite-biotite Gneiss in the South-East Highlands and it* 

 accompanying Metamorphism" : Quart. Journ. Geol. Soc, vol. xlix, p. 330. 

 5 Quart. Journ. Geol. Soc, vol. xlvii, p. 72. 

 « Ann. Rep. Geol. Surv., 1893, p. 266 ; 1895, p. 25 ; 1896, p. 27. 



