September 23, 1915] 



NATURE 



'03 



and dimplings of the crust, which are already worn 

 down or inhlled respectively by denudation and de- 

 position. Their present forms and places record the 

 last movements of the earth-storm, just as a buckled 

 tramway-rail records the passage of an earthquake. 

 How shall we gauge to-day the intensity of their rise 

 and fall ? 



In the case of the city devastated by an earthquake, 

 the d<5bris is cleared away, and our descendants in 

 time discover the distorted rails beneath the healing 

 mantle of new grass. Will they realise from this 

 alone the preliminary tremors, the sudden arrival of 

 the culminating vibration, the shock that overcame 

 the elasticity of the crust beneath them, and then the 

 gradual establishment of the conditions under which 

 they have passed their peaceful lives? The crumpled 

 wreckage lies there in evidence before them ; but how 

 will they distinguish the work of a few stormy seconds 

 from that due to the gentle earth-creep of a century? 



(2) Regions of Subsidence. 

 It was probably E. Suess who brought home to most 

 of us the importance of regions of subsidence in defin- 

 ing the lowlands and the sea-basins from the up- 

 standing masses of the crust. While one region may 

 be folded, another may be broken into blocks ; and the 

 two types of movement, that due to tangential thrust- 

 ing and that due to vertical uplift and down-faulting, 

 may appear in the same region and may alike play 

 their part in producing a lowering of large areas. 

 \ The domes and dimples that occur beyond the region 

 ' of acute crumpling may be intensified into fault-blocks 

 1 by fracture of their boundaries. If catastrophes are 

 •, possible during uplift, we may look for them also 

 I during subsidence. 



; The cutting-up of mountain-chains by transverse 

 fractures has resulted in the loss of huge blocks be- 

 neath the sea. In such cases it is clear that faulting 

 has run a long way ahead of denudation. All trough- 

 valleys, which are often called, somewhat mislead- 

 ingly, rift-valleys, raise the same questions as to the 

 nature of the steps by which they have been produced. 

 The Rhine Vale, one of the most closely studied ex- 

 amples, dropped 8000 ft. within the limits of Oligocene 

 time. It is improbable that the numerous faults now 

 traceable operated with concerted gentleness. 



Abruptness of certain Geographical Changes. — River- 

 capture. 



There is a totally different class of terrestrial pheno- 

 mena which lends itself also to speculation, or to 

 ! that imaginative faculty, proper to our Section, which 

 enables the geologist to reconstruct. Geographers 

 have taught us to speak lightly of river-diversion and 

 river-capture, and to treat them as frequent occur- 

 rences in the history of existing lands. It is interest- 

 ing to Inquire what this process on a large scale may 

 involve. 



The draining of the Ragunda lake in Sweden ^^ in 

 1796, by the rapid cutting of a ravine 100 ft. deep In 

 a soft barrier, shows how many of our Glacial lakes, 

 dammed by morainic matter, may have excavated their 

 outlet gorges and run dry in the course of a few hours. 

 The history of the temporary lake behind the Gohna 

 landslip, so brilliantly studied by our vice-president. 

 Sir Thomas Holland, ^^ provided a lesson both in hill- 

 destruction and catastrophic flooding. The diversion 

 of the Colorado River, however, in 1905, into the 

 sluice leading to the Salton SInl> gives us a definite 

 illustration of river-capture. The " New River " thus 



- 21 See especially H. W. Ahlmann, " Ragundasjon"! Geomorfologoi," 

 Siierines Geol. Untiersok, 191 5 ; also Ahlmann, Carlzon, and Sandeeren. 

 ' Qnaternary History of the Ragunda Region, J.lmtland," Geol. Fdren. 

 rhandl., vol. xxxiv. (iqtj), p- 343- 



-- Records Geol. Surv. India, voi. xxvii. (1894), p. 55, and Naturk, 

 vol. 1., p. 501. 



NO, 2395, VOL. 96I 



produced in the depression to the north-west of 

 Calexico cut a valley 70 ft. deep through the agricul- 

 tural land that it was meant to serve, and worked the 

 head of this valley backward at the rate of a third 

 of a mile a day. 



One of the most remarkable instances of river- 

 diversion in the European record is that of the waters 

 from the north side of the central Alps. At the close 

 of the Pliocene period, the north slopes of the St. 

 Gothard mass and the Bernese Alps, supplying the tor- 

 rents of the Reuss-Aar-Saane system, drained across the 

 hummocky land near Bale and sent their waters over 

 to the Doubs. The great Rhine-trough drained south- 

 ward, and Its streams formed tributaries of the Alpine 

 flow near Bale. The Mainz basin, however, which 

 was infilled by Lower Pliocene alluvium, became 

 tapped by the head of a river that had long run north- 

 ward froni the Hunsriick-Taunus range. This river 

 is the Rhine that we know north of Coblenz, and its 

 alluvium was then spread out where the sea now 

 stretches between Holland and the Yorkshire coast. 

 Its mature valley Is still traceable^' above the present 

 stream-cut In the hills. This river could have no 

 direct Influence on the course of the drainage from 

 the Alps. But the bulging of the land at the north 

 end of the Juras still continued. .-Xs the text-books 

 remark with some complacency, the Burgundian gate 

 was closed, and the river that had previously crossed 

 westward was diverted northward to the Rhine-trough. 



Can we exactly picture what this means? The 

 whole Reuss-.^ar-Saane system "on some particular 

 day began to flow northward along the far older 

 tectonic trough, carving away the infilling of detritus, 

 washing back tree-stems that were floating quietly 

 from the Lake of Mainz on their way to the Mediter- 

 ranean, and finding, when It reached that lake, a 

 notch sufficiently low for its escape across the Huns- 

 riick-Taunus range. An enormous body of water was 

 thus added to that which had formed In Pliocene times 

 a mature valley across these hills." =^* The system in- 

 dicated above, representing the flow from a hundred 

 miles of snow-clad mountains, must have made a re- 

 markable change in the stream across the .^rmorican 

 hills. When the Alpine water arrl\'ed at the Mainz 

 basin, and found Its w^ay into the notch formed by 

 the Pliocene Rhine, it poured down upon the forest- 

 covered delta-land. The changes that have occurred 

 In the unconsolidated ground of Holland in historic 

 times furnish some picture of what must have hap- 

 pened In the prehistoric delta of the Rhine. Land was 

 suddenly built up at some points, islands were carved 

 out at others, and the effects of the catastrophe must 

 have been still manifest when the Scandinavian ice- 

 sheet began to invade the mud-flats from the north. 



The capture of a large river may be Illustrated by 

 the story of the Vistula. This noble stream represents 

 in a remarkable way the drainage of 190 miles of the 

 Carpathians. All this water becomes concentrated, at 

 the apex of a reversed river-fan, at the east end of 

 the Kielce hills, and it is probable that the upper 

 Vistula was driven to join the San by the advancing 

 ice-front of the Riss age, and that both rivers then 

 escaped southwards. The joint waters were again 

 held up when the Fennoscandlan Ice rested along the 

 line marked by the Baltic Heights, and it is well 

 known that a great river flowed westward^ along the 

 stagnating ice-front where now the marshes of the 

 Netze mark its course. As the ice-front shrank back- 

 ward, towards the Baltic basin,'* streams flowed down 



i3 W. M. Davis, " Die erklarende Beschreibung der Landformen " (1911), 



24 fi. A. J. Cole. "The Growth of Fnrope " (1914), p. 109. 



25 R. Lepsius (Gtohgie von Deutschland, pt. 2, p. 511) urges that the 

 s-nking of the floor of northern Europe led to this northward trend of the 

 streams. 



