350 



NATURE 



[August ii, 1892 



Two kinds of crustal movement, as we have seen, are recog- 

 nized by geologists. Sometimes the crust appears to rise, or, 

 as the case may be, to sink over wide regions, without much 

 disturbance or tilting of strata, although these are now and 

 again more or less extensively fractured and displaced. It may 

 conduce to clearness if we speak of these movements as 

 regional. The other kind of crustal disturbance takes place 

 more markedly in linear directions, and is always accompanied 

 by abrupt folding and mashing together of strata, along with 

 more or less fracturing and displacement. The plateau of the 

 Colorado has often been cited as a good example of regional 

 elevation, where we have a wide area of approximately hori- 

 zontal strata apparently uplifted without much rock-disturbance, 

 while the Alps or any other chain of highly flexed and con- 

 voluted strata will serve as an example of what we may term 

 axial or linear uplifts. It must be understood that both regional 

 and axial movements result from the same cause — the adjust- 

 ment of the solid crust to the contracting nucleus — and that the 

 term ekvation, therefore, is only relative. Sometimes the 

 sinking crust gets relief from the enormous lateral pressure to 

 which it is subjected by ridging up along lines of weakness, and 

 then mountains of elevation are formed ; at other times, the 

 pressure is relieved by the formation of broader swellings, when 

 wide areas become uplifted relatively to surrounding regions. 

 Geologists, however, are beginning to doubt whether upheaval of 

 the latter kind can affect a broad continental area. Probably in 

 most cases, the apparent elevation of continental regions is only 

 negative. The land appears to have risen because the floor 

 of the oceanic basin has become depressed. Even the smaller 

 plateau-like elevations which occur within some continental 

 regions may in a similar way owe their dominance to the sinking 

 of contiguous regions. 



In the geographical development of our land, movements of 

 elevation and depression have played an important part. But 

 we cannot ignore the work done by other agents of change. If 

 the orographical features of the land everywhere attest the 

 potency of plutonic agents, they no less forcibly assure us that 

 the intqualities of surface resulting from such movement are 

 universally modified by denudation and sedimentation. Elevated 

 plains and mountains are gradually demolished, and the hollows 

 and depressions of the great continental plateau become slowly 

 filled with their detritus. Thus inland seas tend to vanish, inlets 

 and estuaries are silted up, and the land in places advances sea- 

 ward. The energies of the sea, again, come in to aid those of 

 rain and river, so that under the combined action of all the 

 superficial agents of change, the irregularities of coast-lines 

 become reduced, and, were no crustal movement to intervene, 

 would eventually disappear. The work accomplished by those 

 agents upon a coast-line is most conspicuous in regions where 

 the surface of the continental plateau is occupied by compara- 

 tively shallow seas. Here full play is given to sedimentation 

 and marine erosion, while the latter alone comes into promi- 

 nence upon shores that are washed by deeper waters. When 

 the coast-lines advance to the edge of the continental plateau, 

 they naturally trend, as we have seen, for great distances in 

 some particular direction. Should they preserve that position, 

 undisturbed by crustal oscillation, for a prolonged period of 

 time, Ibey will eventually be cut back by the sea. In this way a 

 shelf or terrace will be formed, narrow in some places, broader in 

 others, according to the resistance offered by the varying character 

 of the rocks. But no long inlets or fiords can result from such 

 action. At most the harder and less readily demolished rocks 

 will form headlands, while shallow bays will be scooped out of 

 the more yielding masses. In short, between the narrower and 

 broader parts of the eroded shelf or terrace a certain proportion 

 will tend to be preserved. As the shelf is widened, sedimenta- 

 tion will become more and more effective, and in places may 

 come to protect the land from further marine erosion. This 

 action is especially conspicuous in tropical and subtropical 

 regions, which are characterized by well-marked rainy seasons. 

 In such regions immense quantities of sediment are washed 

 down from the land to the sea, and tend to accumulate along 

 shore, forming low alluvial flats. All long-established coast- 

 lines thus acquire a characteristically sinuous form, and perhaps 

 no better examples could be cited than those of Western 

 Africa. 



To sum up, then, we may say that the chief agents concerned 

 in the development of coast-lines are crustal movements, sedi- 

 mentation, and marine erosion. All the main trends are the 

 result of elevation and depression. Considerable geographical 



NO. 



I 189, VOL. 46 J 



changes, however, have been brought about by the silting up 

 of those shallow and sheltered seas which, in certain regions, 

 overflow wide areas of the continental plateau. Throughout all 

 the ages, indeed, epigene agents have striven to reduce the 

 superficial inequalities of that plateau, by levelling heights and 

 filling up depressions, and thus, as it were, flattening out the 

 land surface and causing it to extend. The erosive action of the 

 sea, from our present point of view, is of comparatively little 

 importance. It merely adds a few finishing touches to the 

 work performed by the other agents of change. 



A glance at the geographical evolution of our own continent 

 will render this sufficiently evident. Viewed in detail, the 

 structure of Europe is exceedingly complicated, but there are 

 certain leading features in its architecture which no profound 

 analysis is required to detect. We note, in the first place, that 

 highly disturbed rocks of Arch^ean and Palaeozoic age reach 

 their greatest development along the north-western and 

 western borders of our continent, as in Scandinavia, the 

 British Islands, North-west France, and the Iberian penin- 

 sula. Another belt of similarly disturbed strata of like 

 age traverses Central Europe from west* to east, and is 

 seen in the South of Ireland, Cornwall, North-west France, the 

 Ardennes, the Thiiringerwald, the Erzgebirge, the Riesenge- 

 birge, the Bohmerwald, and other heights of Middle and 

 Southern Germany. Strata of Mesozoic and Cainozoic age 

 rest upon the older systems in such a way as to show that the 

 latter had been much folded, fractured, and denuded before 

 they came to be covered with younger formations. North 

 and north-east of the central belt of ancient rocks just referred 

 to, the sedimentary strata that extend to the shores of the Baltic 

 and over a vast region in Russia, range in age from Palaeozoic 

 down to Cainozoic times, and are disposed for the most part 

 in gentle undulations— they are either approximately horizontal 

 or slightly inclined. Unlike the disturbed rocks of the mari- 

 time regions and of Central Europe, they have obviously been 

 subjected to comparatively little folding since the time of their 

 deposition. To the south of the primitive backbone of Central 

 Europe succeeds a region composed superficially of Mesozoic 

 and Cainozoic strata for the most part, which, along with under- 

 lying PalcEozoic and Archaean rocks, are often highly flexed and 

 ridged up, as in the chains of the Jura, the Alps, the Car- 

 pathians, &c. One may say, in general terms, that throughout 

 the whole Mediterranean area Archaean and Palaeozoic rocks 

 appear at the surface only when they form the nuclei of moun- 

 tains of elevation into the composition of which rocks of younger 

 age largely enter. 



From this bald and meagre outline of the general geological 

 structure of Europe, we may gather that the leading orographical 

 features of our continent began to be developed at a very early 

 period. Unquestionably the oldest land areas are represented 

 by the disturbed Arch^an and Palaeozoic rocks of the Atlantic 

 sea-board and Central Europe. Examination of those tracts 

 shows that they have experienced excessive denudation. The 

 Archaean and Palaeozoic masses, distributed along the margin of 

 the Atlantic, are the mere wrecks of what, in earlier ages, must 

 have been lofty regions, the mountain-chains of which may well 

 have rivalled or even exceeded in height the Alps of to day. 

 They, together with the old disturbed rocks of Central Europe, 

 formed for a long time the only land in our area. Between the 

 ancient Scandinavian tract in the north and a narrow interrupted 

 belt in Central Europe stretched a shallow sea, which covered 

 all the regions that now form our Great Plain ; while immedi- 

 ately south of the central belt lay the wide depression of the 

 Mediterranean — for as yet the Pyrenees, the Alps, and the 

 Carpathians were not. Both the Mediterranean and the Russo- 

 Germanic sea communicated with the Atlantic. As time went 

 on, land continued to be developed along the same lines, a result 

 due partly to crustal movements, partly to sedimentation. Thus 

 by and by the relatively shallow Russo-Germanic sea became 

 silted up, while the Mediterranean shore-line advanced south- 

 wards. It is interesting to note that the latter sea, down to the 

 close of Tertiary times, seems always to have communicated 

 freely with the Atlantic, and to have been relatively deep. The 

 Russo-Germanic sea, on the contrary, while now and again 

 opening widely into the Atlantic, and attaining considerable 

 depths in its western reaches, remained on the whole shallow, 

 and ever and anon vanished from wide areas to contract into a 

 series of inland seas and large salt lakes. 



Reduced to its simplest elements, therefore, the structure of 

 Europe shows two primitive ridges— one extending with some 



