September 19, 1901] 



NATURE 



513 



mena in the field, viz., that under the influence of horizontal 

 compression or earth-creep the rocks in that region behaved like 

 brittle rigid bodies which snapped across, were piled up and 

 driven westwards in successive slices. But, further, these dis- 

 placements were accompanied by differential movement of the 

 materials which resulted in the development of new structures. 

 These phenomena culminate along the belt of rocks in immediate 

 association with the Moine Thrust, where the outcrop of that 

 thrust lies to the east of a broad belt of displaced materials. 

 There, Lewisian gneiss, Torridon Sandstone, and Cambrian 

 quartzite are sheared and rolled out, presenting new divisional 

 planes parallel with that of the Moine Thrust. The Lewisian 

 gneiss shades into flaser gneiss and schist, and ultimately passes 

 into a banded rock like a platy schist. The pegmatites show 

 fluxion structure with felspar "eyes" like that of the rhyolites. 

 At intervals in these zones of highly sheared rocks, phacoidal 

 masses of Lewisian gneiss appear, in which the pre-Torridonian 

 structures are not wholly effaced. The sills of camptonite and 

 felsite intrusive in the Cambrian rocks become schistose and 

 together with the sediments in which they occur appear in a 

 lenticular form. All these mylonised rocks show a characteristic 

 striping on the divisional planes, due to orientation of the con- 

 stituents in the direction of movement. 



Still more important evidence in relation to the question of 

 regional metamorphism is furnished by the Torridon Sandstone. 

 In the case of the basal conglomerate the pebbles have been 

 flattened and elongated,, and a fine wavy structure has been 

 developed in the matrix. In the district of Ben More, Assynt 

 pl.anes of schistosity, more or less parallel with the planes of the 

 Ben More Thrust, pass downwards from the Torridon con- 

 glomerate into the underlying gneiss. Both have a common 

 foliation irrespective of the unconformability between them. 

 Again, along the great inversion south of Stromeferry, foliation 

 has been developed in the Torridon conglomerate and overlying 

 Lewisian gneiss, parallel to the plane of the Jloine Thrust. The 

 Torridon grits and sandstones south of Kinlochewe and between 

 Kishorn and Loch Alsh are similarly affected by the post-Cam- 

 brian movements. Mr. Teall has shown that the quartz grains 

 have been drawn out into lenticles and into thin folia that wind 

 round " eyes " of felspar. A secondary crypto-crystalline 

 material has been produced, sericitic mica appears in the 

 divisional planes, and in some instances biotite is developed. 

 In short, he concludes that in these deformed Torridonian sedi- 

 ments there is an approximation to the crystalline schists of the 

 Moine type. The stratigraphical horizon of these rocks can be 

 clearly proved. The subdivisions of the Torridon Sandstone 

 have been recognised in those displaced masses which lie to the 

 east of the Kishorn Thrust and to the west of the Moine Thrust. 

 It is worthy of note also that in the belt of highly sheared gneiss 

 south of Stromeferry that comes between the Torridonian in- 

 version in the west and the Moine Thrust on the east, Mr. 

 Peach has found folded and faulted inliers of the basal division 

 of the Torridon Sandstone that have a striking resemblance to 

 typical Moine schists. 



Regarding the age of these post-Cambrian movements, it is 

 obvious that they must be later than the Cambrian limestone 

 and older than the Old Red Sandstone, for the basal con- 

 glomerates of the latter rest unconformably on the eastern 

 schists and contain pebbles of basal quartzite, pipe-rock, lime- 

 stone, and dolomite derived from the Cambrian rocks of the 

 North-west Highlands. 



East of the Moine Thrust or great line of displacement ex- 

 tending from EriboU to Skye, we enter the wide domain of the 

 metamorphic rocks of the Highlands, a region now under in- 

 vestigation, and which presents difficult problems for solution. 

 Two prominent types of crystalline schists (Caledonian series, 

 Callaway, and Moine schists of the Geological Survey) have 

 been traced over wide areas in the counties of Sutherland, Ross, 

 and Inverness, and across the Great Glen to the northern slopes 

 of the Grampians. Consisting of granulitic quartzose schists 

 and muscovite-biotite schist or gneiss, they appear to be of sedi- 

 mentary origin, though crystalline. They are associated with 

 recognisable masses of Lewisian gneiss covering many square 

 miles of ground and presenting many of the structures iso 

 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 granu- 

 litic structure like that of the adjoining Moine schists (" Sum- 



NO, 1664, VOL. 64] 



mary of Progress of the Geological Survey for 1897," p. 37). 

 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 sedi- 

 mentary 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 sedi- 

 ments, as proved by Mr. Hill in the neighbourhood of Loch 

 Awe ("Annual Report of the Geological Survey for 1893," 

 p. 265). 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 metamorphism ; 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 iSgi {Quart. 

 Joiini. Geol. Soc.,vo\. xlvii. p. 72), evidence has been obtained 

 ("Annual Report of the Geological Survey for 1S93," p. 266; 

 for 1S95, p. 25 ; for 1896, p. 27) 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 disruption 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 Dal- 

 radian 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 uncon- 

 formably by sedimentary strata which have been correlated with 

 the lower and middle divisions of the Torridon Sandstone. Un- 

 fortunately the sequence ends here, as both the gneiss and over- 

 lying 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 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 

 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 (" Summary of Progress 

 for 1S99," p. 66). 



From the foregoing data it will be seen that much uncertainty 

 prevails regarding the age and structural relations of the meta- 

 morphic 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 



1 " On Foliated Granites and their Relations to the Crystalline .Schists 

 in Eastern Sutherland ' [Quart. Journ. Gcol. Sue, vol. lii. p. 633). 



2 " Intrusion of Muscovite-biotite Gneiss in the South-east Highlands 

 and its accompanying Metamorphism" {Quart. Journ. Geol. Soc.^ vol. 

 xlix. p. 330). 



