284 



KNOWLEDGE. 



[December 1, 1897. 



plains of Argentina, the same efifacement of surface- 

 features may be seen — the continents disappear, as it 

 were, beneath the encroachment of a flood. Can we 

 believe, then, that the heart of a continent is more stable 

 than its coastline ? Without upward movements, without 

 the formation of new folds in the earth's crust, continental 

 land must be reduced to stretches of desert, above which 

 no highland will remain to cause condensation of the air 



Fra. 2. — Toblac'U and the Vallcj of the Rien/. in Tyrol, showing 

 the formation of a plain by deposition of detritus at tlie foot of 

 mountains. 



blown inward from the ocean ; or to desolate regions of 

 salt marshes, the evaporating relics of former inland 

 lakes ; while here and there a line of mounds, coated with 

 conglomerates decaying into sands, will mark the site of 

 our more famous mountain- chains. 



How soon such a result might be arrived at is a matter 

 of speculation rather than of acci;rate inquiry. We are 

 still in the position of old John Playfair,* who wrote in 

 1802, " It has been supposed that the Pyrenees diminish 

 about ten inches in a century ; but what confidence is to 

 be put in this estimate I am unable to determine." It is 

 certain, however, that the decay is rapid compared with 

 what we know of geological time. The converse of this 

 statement is that the continents must themselves be young. 



Examination of the geological maps that have been 

 drawn up by various Surveys will show us that any large 

 surface of the earth, where not buried in recent detritus, 

 is of a very complex character. In the heart of a continent, 

 as on its margins, we find expressed the sum of a series of 

 earth-movements which have been going on since the 

 first consolidation of the crust. Evidences of elevation 

 alternate with those ol depression ; and even such oceanic 

 deposits as much of the Upper Chalk I and the Num- 

 mulitic Limestone are found stretching across continental 

 plateaus, or infolded in the crests of mountains. J It is 

 certain that the prominent features of our continents, 

 the sources of our most extensive alluvial deposits, are of 

 comparatively recent geological age. Here and there, we 

 have broad masses, like Scandinavia and Scotland, that 

 seem to have been elevated, high and dry, for an abnormal 

 length of time ; and here and there, in the interior of our 

 continents, knobs and bosses of the ancient floor stand up 

 above the Secondary and Tertiary deposits. But the 



* " Illustrations of the Huttonian Theory," p. 380. 

 + See W. F. Hume, "The Genesis of the Challi," Pruc. Oeol. 

 ^«oc.,Vol. XIII., p. 241. 

 I Compare " The Student's Lyell," eJ. hy J. W. Ju Id, p. 230. 



lowlands in which the gravels and the dust accumulate 

 have often emerged only recently from beneath the sea. 

 Deltas have been converted into a foundation for talus- 

 fans, and marine strata, barely consolidated, have become 

 a prey to the distributing action of the wind. Thus the 

 geological map of Russia shows the large area occupied 

 respectively by the marine beds of the Caspian area, 

 stretching northward, and those representing a recent 

 Arctic inflow, stretching southward. These beds are 

 barely older than the dawn of human history ; yet they 

 indicate that the Danube and the Black Sea then drained 

 out northward, into an arm of the Polar ocean. The sea, 

 in fact, then covered a large midland area of the joint 

 Eurasian continent. The lowlands of the Amazon,! more- 

 over, in the very centre of South America, are occupied by 

 some of the most recent marine deposits of the continent — 

 older than those of the Caspian steppe, but still of " Later 

 Tertiary " age ; and these deposits are now one thousand 

 five hundred miles inland from the Atlantic Ocean. 



Piof. Suess, at the end of the first volume of his 

 " Antlitz der Erde," has summaiised his investigations 

 in!o the characters of the existing continents ; and we may 

 rapidly come to the conclusion that there are no special 

 buttresses supporting our great land-masses, but that 

 fracture of the crust may let in the ocean over half a 

 continent at a time. Thus the tablelands of South and 

 Central Africa appear to date from the Carboniferous 

 period ; but they are grouped witb Madagascar, India, and 

 the desert-lands of North Africa and Arabia, to form one 

 of the great structural divisions of the earth. The Indian 

 Ocean entered this continental area in comparatively 

 recent times ; and Dr. Oregory ; insists that great changes 

 in the relative levels of the areas examined by him in 

 British East Africa have occurred during the Later Tertiary 

 periods. He confirms Prof. Suess's opinion that the 

 " Great Rift Valley," from the Zambesi to the .Jordan, is 

 an elongated region of recent subsidence. 



Fig-. 3. — Landscape iu the plain oi tlie IJanube, near .Straubinsr, 

 Bavaria, showing the character of a great area of aUuvial accumula- 

 tion. (From a photograph by Jfr. A. \V. Bawtree. F.L.S.) 



Suess divides " Eurasia " from the broken and faulted 

 contment of " Indo-Africa" by the zone of intense folding 

 that dates mainly from Upper Miocene times. From 



* "Carte geol. de la Russie d'Europe, editea par le Comite geolo- 

 titiue," 1892. 



+ C. B. Brown, " On the Tertiary Deposits of the Solimo^-s and 

 Javary Kivers," Quart. Joiini. Geol. Soc, Vol. XXXV., pp. 81 aul 83. 



I " The Great Rift Valley," 1896. 



