'KIVEK VALLEYS OP THE WEST AFRICAN CONTINENT^ ETC, 157 



Geological Conditions. — The valleys which descend from 



the Alps are eroded through strata of successive geological 



ages in an ascending series as we approacli the coast ; thus 



the valleys of the Basagno, Polcevera, the Merula, and 



Arma are eroded through Eocene strata, but the Roida 



valley is eroded through both Eocene, Miocene, and 



Pliocene formations. And as this valley (as well as the 



others) is prolonged under the waters of the oMediterranean, 



it follows that the valley is of a ncAver geological epoch than 



the Pliocene, through which it is eroded; in other words, it 



is of Post-Pliocene age. Tliis exactly brings us to tiie epoch 



to which I had ventured to refer the elevation of Western 



Europe and the formation of the sub-oceanic river valleys, and 



it is eminently satisfactory to have such confirmatory evidence. 



Such an elevation of the Alpine region as that determined 



by Professor Issel, namely, some 3.000 feet as a minimum, 



during the Post-Pliocene or Glacial period, would go a long 



way to account for the great accumulation of snoAv, and the 



extension of the glaciers beyond their present limits, which 



Ave know to have taken place at that epoch. But the 



elevation has been doubtless even greater, as inferred from 



the depth to Avhich the sub-oceanic valleys have been traced, 



along the Avestern European coast, as Avell as in those of the 



Ebro and Rhone above described. The soundings made by 



the officers of the "Washington" may not haA'-e extended 



far enough out to sea to permit Professor Issel to trace the 



river A^alleys further than the 900-metre contom-, but the 



valleys themselves probably descend to greater depths. 



involving conditions of greater elcA^ation, and subsequent 



depression. 



General Conclusions — Long dm^ation of the Post-Pliocene 

 Period. — From the above statements it Avill be apparent that 

 the phenomena of terrestrial elevation, resulting in the 

 formation of submarine valleys, and of subsequent depres- 

 sion, resulting in their submergence, Avhich have been 

 demonstrated in the case of Western Europe and the British 

 Isles, are fully borne out by the soundings off the coast of 

 Africa and Avithin the Mediterranean. They are all in 

 harmony, and point to an elevation of the lands bordering 

 the Atlantic to the extent of about 7,000 to 8,000 feet, and 

 subsequent depression during a period geologically " Recent " 

 or " Post-Tertiary." Professor Issel's determination of the age 

 of these great moA^ements, based on geological grounds, 

 coincides Avith ray OAvn, drawn from other sources. And in 



