May ir, i88S.] 



SCIENTIFIC NEAATS. 



449 



of lMptv$y Hecture^, etc* 



GEOLOGICAL SOCIETY OF LONDON. 

 At the meeting held on April 25th, 1888, Mr. W. T. 

 Blanford, LL.D., F.R.S., President, in the chair, the 

 following communications were read : — 



" Report on the Recent work of the Geological 

 Survey in the North-west Highlands of Scotland, based 

 on the field-notes and maps of Messrs. Peach, Home, 

 Gunn, Clough, Hinxman, and Cadell." Communicated 

 by A. Geikie, LL.D., F.R.S., F.G.S., Director-General. 



At the outset a review was given of the researches of 

 other observers, in so far as they forestalled the con- 

 clusions to which the geological survey had been led. 

 Reference was made to the observations of Macculloch, 

 Hay, Cunningham, C. W. Peach, and Salter; to the pro- 

 longed controversy between Sir Roderick Murchison and 

 Professor Nicol ; to the contributions of Hicks, Bonney, 

 Hudleston, Callawaj', Lapworth, Teall, and others. It 

 was shown that Nicol was undoubtedly right in main- 

 taining that there was no conformable sequence from the 

 fossiliferous quartzites and limestones into the eastern 

 schists. It was also pointed out that the conclusions 

 of Professor Lapworth regarding the nature and origin 

 of the eastern schists involve an important departure 

 from Nicol's position, and are practically identical with 

 those obtained independently by the Geological Survey. 



The results of the recent survey work among the 

 Archaean rocks may be thus summarised : — (i) the 

 eruption of a series of igneous rocks of a basic type in 

 which pegmatites were formed ; (2) the development 

 of rude foliation in these masses, probably by 

 mechanical movement, and their arrangement in gentle 

 anticlines and synclines, the axes of which generally run 

 N.E. and S.W. ; (3) the injection of igneous materials, 

 mainly in the form of dykes, into the original gneisses, 

 composed of (a) basalt rocks, (b) peridotites and 

 palaeopicrites, (c) microline-mica rocks, (rf) granites ; (4) 

 the occurrence of mechanical movements giving rise to 

 disruption-lines trending N.W. and S.E., E. and W., 

 N.E. and S.W. ; (5) the effects of these movements on 

 the dykes were to change the basalt-rocks into diorites 

 and hornblende-schists, the peridotites and palasopicrites 

 into talcose schists, the microcline-mica rocks into mica- 

 schists, and the granites into granitoid gneiss ; (6) the 

 effects on the gneiss resulted in the formation of sharp 

 folds trending generally N.W. and S.E., the partial or 

 complete reconstruction of the original gneiss along the 

 old foliation-planes, and finally the development of 

 newer schistosity more or less parallel with the 

 prominent disruption-lines. 



There is an overwhelming amount ot evidence to 

 prove that all these various changes had been super- 

 induced in the Archaean rocks in Pre-Cambrian time. 



After reviewing the facts bearing on the denudation ot 

 the Archaean land-surface, the order of succession and 

 thickness of the Cambrian strata were given, from which 

 it is apparent that the deposits gradually increase in 

 thickness as we pass southwards from Durness to Loch 

 Broom. 



Prior to the deposition of the Silurian sediments the 

 Cambrian strata were folded and extensively denuded. 

 By these means various Cambrian outliers were formed 

 far to the east of the present limits of the formation. 



The order of succession of the Silurian strata along 

 the line of complicated structure from Eriboll to Ulla- 

 pool wras described, reference being made to the further 

 subdivision of the " Pipe-rock " and the Ghrudaidh 

 Limestones (Group I. of Durness seciion). None ot 

 the richly fossiliferous zones of Durness is met with 

 along this line, as they occupy higher horizons. An 

 examination of the fossils recently obtained by the 

 Geological Survey from the Durness Limestones con- 

 firms Salter's conclusions that they are distinctly of an 

 American type, the Sutherland quartzites and limestones 

 being represented by the Potsdam Sandstones and Cal- 

 ciferous Sand Group of North America. 



After a deposition of the limestones, the Cambrian 

 and Silurian strata were pierced by igneous rocks, 

 mainly in the form of sheets, producing important 

 alterations in the sedimentary deposits by contact- 

 metamorphism, the quartzites becoming crystalline, and 

 the limestones being converted into marble. 



When this outburst of volcanic activity had ceased, 

 terrestrial displacements ensued on a stupendous scale. 

 By means of powerful thrusts the Silurian strata were 

 piled on each other, and huge slices of the old Archaean 

 platform, with the Cambrian and Silurian strata resting 

 on it, were driven westwards for miles. With the view 

 of illustrating the extraordinary complications produced 

 by these movements, a series of horizontal sections was 

 described drawn across the line between Eriboll and 

 Ullapool. 



The evidence relating to regional metamorphism was 

 next referred to, from which it is obvious that with each 

 successive maximum there is a progressive amount of 

 alteration in the displaced masses, as the observer passes 

 eastwards to the higher thrust-planes. Eventually the 

 Archaean gneiss is so deformed that the Pre-Cambrian 

 foliation disappears and is replaced by new divisional 

 planes ; the Cambrian grits and shales are converted 

 into schists ; the Silurian quartzites into quartz-schists ; 

 the limestones become crystalline ; the sheets of intru- 

 sive felsite, diorite, and granitoid rock pass into 

 sericite schist, hornblende-schist, and augen-gneiss 

 respectively. 



The researches furnish a vast amount of evidence in 

 support of the theory that regional metamorphism is 

 due to the dynamical and chemical effects of mechanical 

 movement acting on crystalline and elastic rocks. It is 

 also clear that regional metamorphism need not be con- 

 fined to any particular geological period, because in the 

 N.W. Highlands, both in Pre-Cambrian time and after 

 the deposition of the Durness Limestone (Lower 

 Silurian), crystalline schists and gneiss were produced 

 on a magnificent scale. 



" On the Horizontal Movements of Rocks, and the 

 relation of these movements to the formation of Dykes 

 and Faults and to denudation and the thickening of 

 Strata." By William Barlow, Esq., F.G.S. 



The paper commenced with a description of some 

 horizontal movements of rocks caused by gravitation ; 

 and the author quoted Mr. C. E. Dutton's descriptions of 

 the Grand Caiion District, especially noting the fact that 

 between succeeding escarpments the strata dip slightly 

 from the crest of the one below to the foot of the next 

 above, and that whilst the strata of the median parts of 

 each terrace are nearly horizontal, the inclination in- 

 creases as we approach the escarpment of the next higher 

 terrace, and also that Dutton observed indications of a 



