;22 



NATURE 



{August I, 1889 



structure of Scandinavia. The lines of maximum thrust- 

 iplanes can be followed for 100 miles, from the north of 

 •Sutherland into Skye ; but this is only a small part of 

 their extent. They can be picked up again in the west 

 of Mayo and Donegal, a total distance of some 400 miles. 

 That similar lines of movement have affected Scandinavia 

 and produced the distinctive strike of the rocks there can 

 hardly be doubted, so that the total length of disturbed 

 country in North-Western Europe probably exceeds 1600 

 miles, trending in a general north-north east direction. 



How far the influence of the great terrestrial move- 

 ments extended eastwards from what now appears as 

 the belt of maximum disturbance, and what effect it 

 had upon the configuration of the surface, are questions 

 to which as yet no satisfactory answer can be given. 

 It is difficult to suppose that such colossal displacements 

 and fractures of the crust should not have powerfully 

 affected the superficial topography of the time. They 

 may have produced a high mountain range, or a succes- 

 sion of parallel ranges, extending along the north-west of 

 Europe. The existence of some such mass of land is 

 needed to account for the vast piles of sediment of which 

 the Palaeozoic, Secondary, and Tertiary formations have 

 been built up. So great, however, is the antiquity of these 

 terrestrial movements, so continual and gigantic has 

 been the denudation, and so repeated have been the 

 oscillations of level, that the upheaved land has been 

 reduced to the fragments that now form the Highlands 

 and Islands of the west of Ireland, of Scotland, and of 

 Scandinavia. 



It is quite clear that during the disturbances in the 

 north-west region the main thrust came from the east- 

 ward. It will be interesting to discover how far towards 

 the east these disturbances afTected the structure of the 

 rocks beneath. That it reached across the whole breadth 

 of the Scottish Highlands — that is, for a distance of 100 

 to 130 miles — can be conclusively proved. That it ex- 

 tended much further, and embraced within its area the 

 whole of the Silurian regions of the three kingdoms can, 

 I think, be shown to be highly probable. 



To understand this part of the problem it is necessary 

 to consider the structure of the ground immediately to 

 the east of the belt of extreme complication in the North- 

 West Highlands. I have said that the displacements and 

 metamorphism increased in intensity from west to east, 

 until at last, by a final gigantic thrust, a series of recon- 

 structed schists has been driven over rocks whose origin 

 can still be determined. Among these eastern schists it 

 is occasionally possible to detect more or less reliable 

 traces of the original rocks out of the crushing down of 

 which they have been formed. Thus we find in the 

 northern part of the area slices of Archaean and eruptive 

 rocks, and in the south an increasing amount of material 

 which has been derived from the destruction of the red 

 Cambrian sandstones. It is tolerably evident that in the 

 broad band of country which extends from the belt of 

 complication eastwards to the Moray Firth and the line 

 of the Great Glen, and embraces the mountainous tracts 

 of Sutherland, Ross, Western Inverness-shire, and North- 

 western Argyllshire, the lower parts of the geological 

 record are repeated again and again. It is mainly the 

 Archaean platform, with its covering of Cambrian sand- 

 stones, and possibly the lower parts of the Silurian series, 

 which have been broken up, plicated, crushed, and con- 

 verted into the series of crystalline schists that form the 

 picturesque heights of Ben Hope and Ben Klibric south- 

 ward to the Moidart and Morven. Nevertheless, when 

 this wild tract of country comes to be mapped out in 

 detail, there will probably be found intercalated bands of 

 higher formations which have here and there been caught 

 in folds of the lower rocks. 



But when we pass eastwards from the Great Glen into 

 the mountains of Eastern Inverness-shire, Perthshire, 

 and the South- Western Highlands, we encounter a totally 



different series of rocks. Though greatly plicated, dis- 

 located, crushed, and metamorphosed, these rocks can 

 be recognized as unquestionably, in the main, of sedi- 

 mentary origin. They must be many thousands of feet 

 in thickness, including among their members such rocks 

 as conglomerate, pebbly grit, quartzite, black slate, anda- 

 lusite slate, phyllite, mica-schist, fine flaggy gneiss, and 

 limestone, together with intrusive sheets and bosses of 

 various eruptive rocks. Some of the groups of this series 

 can be followed and mapped for long distances with 

 nearly as much ease as the members of a succession of 

 unaltered Palaeozoic or Secondary formations. Thei^e is 

 a belt of limestone, for example, which has been traced 

 by the Geological Survey almost continuously from the 

 coast of Banffshire to the west of Argyllshire, through 

 the very heart of the Highlands — a total distance of not 

 much less than 200 miles. These limestones have for 

 the most part become so thoroughly crystalline, that 

 fossils can hardly be expected to be found in them, 

 though there are occasional less altered portions of rock 

 which may eventually prove to be fossiliferous. The 

 limestones are associated with quartzites and schists, as 

 unaltered limestones are with sandstones aud shales. I 

 cannot myself doubt that they have been formed by the 

 aggregation of the remains of calcareous organisms. 

 The same rocks are prolonged into the north of Ireland, 

 where one of the dark limestones at Culdaff has lately 

 yielded certain bodies which some palaeontologists have 

 declared to be the remains of a coral {Favositcs). The 

 black slates which so closely resemble the dark Carbona- 

 ceous shales of the Lower Silurian region of South Scot- 

 land have afforded in Donegal some curious pyritous 

 markings, strongly suggestive of Graptolites. 



Out of this enormous mass of metamorphosed sedi- 

 mentary strata the Scottish Highlands east of the Great 

 Glen are built up, as well as the region which extends 

 southwards across the north-we^t of Ireland as far as the 

 centre of County Galway. The first question that re- 

 quires an answer with regard to it has reference to its 

 relation to the fossiliferous quartzites and limestones of 

 the north-west. Murchison, who led the way in the in- 

 vestigation of the stratigraphy of the Highlands, believed 

 that the quartzites and limestones of the Central High- 

 lands lay towards the base of the whole series of post- 

 Cambrian rocks, and were the south-eastward extensions 

 of those of Sutherland. But recent investigations throw 

 some doubt on this view, which at the time it was pro- 

 mulgated seemed so natural and simple. We know that 

 the quartzites and limestones of the Central Highlands, 

 so far from being near the bottom of the vast series of 

 schists, are underlain by many thousand feet of other 

 metamorphosed sedimentary strata, and that the actual 

 base is nowhere reached in that region. 



During the last two years, in concert with some of my 

 colleagues of the Geological Survey, I have devoted some 

 time to the task of endeivouring to find the bottom of 

 these crystalline schists of Scotland and Ireland, as a 

 necessary foundation for placing them on their true geo- 

 logical horizon, and at length, this spring, our efforts have 

 been successful beyond our expectations. Last year, in 

 the north-west of the Island of Islay, I found a group of 

 scarcely altered shales, grits, and thin limestones emerg- 

 ing from beneath the black slates which underlie the 

 schists, limestones, and quartzites of that region. So 

 little have these strata suffered from the metamorphism 

 which has affected the rocks lying above and to the east 

 of them, that I quite anticipate that fossils will be found 

 in them. This year, in company with Mr. C. T. Clough, 

 I came upon a somewhat similar group of little-meta- 

 morphosed black slates and grits at the north-east end of 

 the Island of lona. I am hopeful that these strata will 

 yield fossils ; I myself found in them some short black 

 limes, which at once recalled the form and condition of 

 the fragments of the central rods of Graptolites so com- 



