::,6 



NA rURE 



\_Ecb. 2 1, 1884 



tliey have prepared their recent work for the engraver. The staff 

 retained in England will have to coniplete the survey of the 

 superficial deposits, which is so valuable as a basis for the agri- 

 cultural valuation of land, as well as for other ipurpose^. For 

 some years past the mapp'ng of these deposits has advanced 

 simultaneously with that of the rocks underneath them. Two 

 kind-; of maps are supplied to the public, one indicating the 

 superficial accumulations, and therefore invaluable as an agricul- 

 tural map, and the other showing the "solid geology" or older 

 rocks that lie below. The importance of mapping the superficial 

 deposits, however, both from an industrial and scientific point of 

 view, was not recognised until comparatively recently. Over the 

 lirger part of the country, therefore, these deposits are not ex- 

 pressed upon the Survey maps, and it is to the completion of this 

 work that one part of the energy of the staff must now lie 

 directed. It will be desirable also to resume the survey of the 

 coalfields on the :cale of six inches to a mile, which has been 

 temporarily interrupted in order to hasten the completion of the 

 one-inch map. The South Wales coalfield, for example, was 

 mapped some forty years ago, and so much has been done in the 

 interval towards the development of that vast mineral basin that 

 the maps are so antiquated as to be of comparatively little 

 practical value. We learn from the same report that the most 

 important work lying before the Survey in England and Wales 

 is the geological description of the country. As the issue of 

 explanatory pamphlets to accompany the one-inch maps »as not 

 begun until 1857, there is a large area i>f ground of which no 

 published account has been given, except on the maps and sec- 

 tions. Printed explanations of each sheet are now to be supplied, 

 aud from these and all the data in possession of the Survey a 

 series of Memoirs or Monographs is to be compiled which w ill 

 embrace a generalised view of the geological structure and 

 of the minerals and industrial resources of the whole country. 

 It is the fate of geological maps, as well as of other human 

 productions, to get out of date. As the nation has ex- 

 pended so ungrudgingly to carry on a Geological Survey 

 which is acknowledged to stand at the head of the geological 

 surveys of the world, it would be worse than folly to lose the 

 benefit of all this expenditure by allowing the maps to becojie 

 obsolete. New openings are continu dly being made which throw 

 fresh light on what lies beneith u-. It will be the duty of Parlia- 

 ment to see that a permanent staff, which need not be a large or 

 cjstly one, is always retained for the purpose of keeping the 

 maps up to date. Meanwhile it is pleasant to see that the work 

 of this worthy national enterprise is being carried on with vigour, 

 and that its staft' are fully alive to the importance of the duties 

 that still lie before them. 



THE ORIGIN OF THE SCENER V OF THE 

 BRITISH ISLANDS' 

 'T'HE Scottish Highlands must be looked upon as the relics of 

 an ancient tableland cut out of highly crumpled and plicated 

 schists. Among the eastern Grampian; large fragments of the 

 plateau exist at heights of more than 3000 feet, forming wide undu- 

 lating plains terminating here and there at the edge of precipices. 

 In the Western Highlands, the erosion having been more pro- 

 found, the ridges are narrower, the valleys deeper, and isolated 

 peaks are more numerous. It is the fate of a tableland to be 

 eventually cut down by running water into a sy.stem of valleys 

 which are widened and deepened, until the blocks of ground 

 between are sharpened into ridges and trenched into separate pro- 

 minences. The Highlands present us with far advanced stages 

 of this process. In the youngest of British tablelands— that of the 

 volcanic region of Antrim and the Inner Hebrides — we meet with 

 some of the earlier parts of the change. That interesting tract of 

 our islands reveals a succession of basaltic sheets which appear 

 to have spread over the wide valley between the Outer Hebrides 

 and the mainland, and to have reached southw'ards beyond 

 LoujIi Neagh, Its original condition must have been like that 

 of the lava-fields of Idaho and Oregon — a sea-like expanse of 

 black basalt stretching up to the base of the mountains. What 

 may have been the total thickness of basalt cannot be told ; but 

 the fragment remaining in Ben More, Mull, ii more than 3000 

 feel thick. So vast has been the erosion since older Tertiary 

 time that the volcanic plateau has been trenched in every direc- 

 tion by deep glens and arms of the sea, and has been reduced 



' Attract of the third of a course of lectures Riven at the Royal Institution, 

 February 19, by Archibald Geikie, F.R.S., Director-General of the Geo- 

 logical Survey. Cont nued from p. 348. 



to detached islands. It is strange to reflect that all this revolu- 

 tion in the to;)ography has been effected since the soft clays and 

 sands of the London Basin were deposited. 



The intimate relation of a system of valleys to a system of 

 drainage lines, first clearly enunciated by Hutton and Playfair, 

 has received ample illustrations from all parts of the world. 

 Yet the notion is not yet extinct that in same way or other 

 valleys have been as much, if not more, determined by subter- 

 ranean lines of di-location as by superficial erosion. Some 

 favourite dogmas die hard, and though this dogma of fracture 

 has been demolished over and over again, it every now and then 

 reappears, dressed up anew as a fresh contribution to scientific 

 progress. We have only to compare the surface of a much dis- 

 located region with its underground structure, where that has 

 been revealed by mining operations, as in our coal-fields, to see 

 that valleys comparatively seldon, and then only as it were by 

 accident, run along lines of dislocation, but that they everywhere 

 cut across them, and that faults rarely make a feature at the 

 surface, except indirectly by bringing hard and soft rocks against 

 each other. 



In Britain, as in other countries, there is a remarkable 

 absence of coincidence between the main drainage .system 

 and the geological structure of the region. We may infer 

 from this fact that the general surface, before the establishment 

 of the present drainage system, had 1 een reduced to a base-level 

 of denudation under the sea, the original inequalities of con- 

 figuration having been planed off irrespective of structure ; or at 

 least, that the present visible rocks were buried under a mass of 

 later unconformable and approximately level strata, on the un- 

 equally upraised surface of which the present drainage system 

 began to he traced. Where the existing watershed coincides 

 generally with the crest of an anticline, its position has oliviously 

 been fixed by the form of the ground ]>roduced by the plication, 

 though occasionally an anticline may have been deeply buried 

 below later rocks, the subsequent folding of which along the 

 same line would renew the watershed along its previous trend. 

 Where drainage line-; coincide with .structure, they are probably, 

 with few exceptions, of secondary origin ; that is, they have 

 been developed during the gradual denudation of the country. 

 Since the existing watershed and main drainage lines of Britain 

 are so independent of structure, and have been determined 

 chiefly by the configuration of the stirface when once more 

 brought up within the influence of erosion, it may be possible to 

 restore in some degree the general distribution of topography 

 when they were begun. 



One of the most curious aspects of the denudation of Britain 

 is its extraordinary inequality. In one region the framework of 

 the land has been cut dow n into the very Archaean core, while 

 in the immediate vicinity there may be many thousands of feet of 

 younger strata which have not been removed. This inequality 

 must result from difference in total amount of upheaval above 

 the base-line of denudation, combined with difference in the 

 length of exposure to denudation. As a rule the highest and 

 oldest tracts will be most deeply eroded. Much of the denuda- 

 tion of Britain appears to have been effected in the interval 

 between the close of the Carboniferous and end of the Triassic 

 periol. This was a remarkable terrestrial interval, during part 

 of which the climate was so arid that salt lakes were formed 

 over the centre of England. Vet the denudation ultimately 

 accomplished was enormous, thousands of feet of Carboniferous 

 rock being entirely removed from certain areas, such as the site 

 of the present Bristol Channel. An interesting analogy to this 

 condition of things is presented by the Great Basin and adjoining 

 tracts of Western America, where at the present time great 

 aridity and extensive salt-lakes are accompanied by great 

 erosion. 



This deeply-eroded post-Carboniferous land was eventually 

 screened from further degradation, either by being reduced 

 through denudation to a base-level or by being protected by sub- 

 mergence. It was to a large extent covered with Secondary 

 rocks, though the covering of these may have been but thin 

 over what are now the higher grounds. The present lerrestri.al 

 areas emerged at some period later than the Chalk. In England 

 there were three tracts of land^Wales, the Pennine Chain, and 

 the Lake District. The eastern half of the country, covered 

 with Secondary rocks, was probably the last portion to be up- 

 lifted above the sea ; hence the watersheds and drainage lines in 

 that tract may be regarded as the youngest of all. 



The history of .some of the valleys of the country tells the 

 story of the denudation. The Thames is one of the youngest 



