March 29, 1894] 



NA TURE 



519 



and the head of Loch Carron some tracts of Torridon Sandstone 

 have been mapped, where the effects of the great displacements 

 upon the internal structure of the sandstone are well di-^played. 

 In that region, by the effect of these stupendous movements, a 

 wide area of Torridon Sandstone and old gneiss has been 

 inverted and pushed bodily westwards so as now to lie upon the 

 Cambrian formations. This inversion is seen on a great scale 

 in the district of Loch Carron, where the Torridon Sandstone, 

 pushed over the quartzites and limestones, dips eastward for 

 several miles until its base passes under the overturned Lewisian 

 gneiss. The actual inverted unconformable junction of the 

 gneiss and sandstone can stillbe traced in various places, though 

 elsewhere it has been effaced by intense deformation. And not 

 only may the inverted unconformability be recognised, but it 

 can be shown that an overlap of the older parts of the Torridon 

 Sandstone takes place against the uneven surface of the over- 

 lying gneiss. Messrs. Peach and Home, who have mapped 

 this remarkable structure, find that the sandstones have been 

 crushed and have become partially schistose, with a develop- 

 ment of mica and other minerals along the planes of movement ; 

 also that pegmatitic veins of quartz and felspar have been 

 formed by segregation in rents of the strata. 



The same intense subterranean movements have profoundly 

 affected the structure of the overlying masses of old gneiss. As 

 these rocks are followed eastwards for several miles they 

 appear more and more sheared, until at last they are succeeded 

 by siliceous granulitic flagstones, such as have been named by 

 the surveyors " Moine-schists. " On the south side of Loch 

 Carron, Mr. Peach has recently obtained evidence which, if con- 

 firmed by further research, will have an important bearing on 

 the interpretation of these schists, which have hitherto presented 

 a very difficult problem. He finds that from the nature of the 

 schists and their mode of occurrence in that locality, they appear 

 to be altered Torridon Sandstone, which has been caught and 

 enclosed within a great synclinal fold by the mass of old gneiss 

 as it was driven westward. They consist of material similar to 

 that of the undoubted Torridon Sandstone, where it has been 

 affected by the greater movements, and they show in many places 

 that they have been originally pebbly felspathic grits. To the 

 eastward of this zone of probably clastic material other huge 

 masses of the Lewisian gneiss have been pushed up and more 

 or less deformed. 



Eastern or Yotmocr Gneiss. — While the chief part of the 

 working season in the north-west Highlands has been devoted 

 to the prosecution of the critical work in the districts just referred 

 to, some progress has also been made in the mapping of the 

 area of the eastern or younger schists (Moine-schists), which, 

 brought forward by the higher thrust-planes, spread over so 

 large an area of Sutherland and Ross. I have referred above 

 to the extreme difficulty of ascertaining what has been the origin 

 of these schists, and to the suggestive observations of Mr. Peach 

 which may eventually lead to the recognition of these rocks as 

 altered sediments. When the officers of the survey some 

 years ago made a few preliminary traverses across the north of 

 Sutherland, before beginning to map that region in detail, they 

 were disposed to believe that certain belts of coarse, banded, 

 gneisses which appear on the coast, were portions of the old 

 gneiss that had escaped the crushing which produced the peculiar 

 granulitic structure of the so-called Moine-schists. Closer 

 examination and detailed mapping of these rocks have led 

 Mr. Home to modify this view. He regards it as certain that 

 altered sediments form an integral portion of the granulitic 

 schists and gneiss of that district. But he also finds that these 

 schists and gneisses are traversed by abundant belts and veins of 

 foliated and unfoliated granite, showing no cataciastic structure. 

 From these larger portions of granitic material countless minute 

 folia of the same substance have proceeded along the foliation 

 planes of the schists. Hence three distinct types of gneiss have 

 been produced : (i) granitoid gneiss or gneissose granite ; (2) an 

 intermediate type consisting of alternations of granulitic and 

 granitic materials ; (3) well-banded biotite-gneiss. If these 

 Kirktomy gneisses really belonged to the Lewisian system, it 

 would follow that the granitic types thus developed must be 

 later than the granulitic schists and gneisses. If, on the other 

 hand, as is most probable, they are of post-Cambrian age, then 

 we must admit that in rocks of this type petrographical 

 characters cannot be regarded as furnishing by themselves a 

 reliable chronological index. 



Schists of Central and Southern Highlands. — In the Central 

 and Southern Highlands, accumulating evidence, both in the 

 field and, as already stated, from microscopical research, goes 



NO. 1274, VOL. 49] 



to show that the main mas'; of the rocks composing that region are 

 a thick and varied series of sedimentary deposits which, t'ogether 

 with their associated igneous materials, have undergone exten- 

 sive metamorphism. The degree of alteration sometimes reaches 

 a point beyond which the original clastic structures are no longer 

 traceable. But even where this is the case with some members 

 of the series, others are found associated with them which can 

 be recognised, and which indicate the persistence of the several 

 stratigraphical groups. An area where there has been hardly 

 any metamorphism has recently been mapped by Mr. J. B. 

 Hill :in the district of Loch Awe. The rocks m that tract 

 consist of grits, phyllites and limestones, which in their un- 

 altered condition resemble ordinary Paljeozoic sedimentary strata. 

 These have been traced by Mr. Hill continuously into the crys- 

 talline schists of the central Highlands. 



Much attention has recently been given to the eruptive rocks 

 associated with the schists of ttie central and southern Highlands. 

 Some of these are dark basic sills, which were injected before 

 the plication and metamorphism of the surrounding rocks, 

 others are later granitic intrusions, probably of difTerent epochs 

 of eruption. That a gradation from basic to acid composition 

 within the same eruptive mass may sometimes be detected, in- 

 dicating probably the order of consolidation of the component 

 materials of an igneous protrusion, has been well shown by 

 recent work of Messrs. Dakyns and Teal). Mr. Barrow's work 

 in Forfarshire has brought to light the existence of a vast number 

 of comparatively small bosses, veins, or lenticles of a granite, 

 with both white and black mica, and usually showing a more or 

 less distinctly foliated structure. He has also found numerous 

 intrusions of a biotite gneiss. Both these rocks are accompanied 

 by an alteration of the surrounding schists. 



ytirassic. — In pursuance of the plan of field-work sketched in 

 previous reports, Mr. H. B. Woodward was stationed in the 

 Isle of Raasay, for the purpose of mapping the various members 

 of the Jurassic series there exposed. He had nearly completed 

 this survey when the short season came to an end, having 

 traced the limits of the Great and Inferior Oolite, the Upper, 

 Middle, and Lower Lias, and the Red Rocks underlying the Lias 

 which are probably of Triassic age. In the course of his work he 

 discovered a hitherto unsuspected bed of Oolitic ironstone in the 

 upper part of the Middle Lias. A thickness of -i. feet 6 inches 

 was seen by him, but the bed may possibly be a little thicker. 

 In geological position this bed corresponds with the well known 

 Cleveland iron-ore and other seams. An analysis, made by Mr. 

 A. Dick, junr., under the superintendence of his father Mr. 

 Allan Dick, who made in 1856 one of the earlier analyses of 

 the Cleveland ore, showed the Raasay stone to contain a little 

 more than 30 per cent, of metallic iron, the proportion in the 

 Cleveland ore ranging from 30 to a little above 33 per cent. 



Glacial Deposits. — In Scotland the mapping of the superficial 

 deposits has gone on simultaneously with that of the solid rocks 

 underneath, and in some parts of the country, where these 

 deposits are especially complicated, the progress of the survey- 

 ing is necessarily somewhat retarded. In the north-west 

 Highlands some singular evidence has been obtained as to the 

 thickness and flow of the great ice-sheet, at what seems to have 

 been the time of maximum glaciation. Not only are the sides of 

 the mountains well ice-worn, but there is evidence that the frag- 

 ments of the characteristic Moine-schists of the interior of 

 Sutherland and Ross have been carried up westward across 

 the great ridges, thus proving that the axis of movement of 

 the ice did not coincide with the present watershed of the 

 country. 



Some interesting relics of the later or valley glaciers have 

 been mapped at Loch Torridon. Mr. Hinxman has traced the 

 moraines down to the latest raised beaches of the west cor.st, 

 Mr. Peach finds evidence that in the north of Sutherland glaciers 

 continued to shed their moraines in the sea at the time of the 

 formation of the 50-feet Raised Beach. Some of the high-level 

 terraces of the River Naver in that district appear to have been 

 formed between the edge of the ice and the side of the valley 

 at a time when a glacier passed down the bed of the valley. 

 Successive terraces, formed in this way as the glacier shrank 

 backwards, may never have reached across the valley, though 

 similar shelves of gravel may now occur on either side. 



Ireland. 



The most important recent work of the staff of the survey of 



Ireland has been a re-examination of certain portions of the 



country with the view of determining how far the gneisses and 



schists represented on the maps could be correlated with those 



