Trof. S. Sjogren — Valleys of the Caucasus. 397 



traverses the prevailing strike of the stratification, and in their 

 upper valleys flow over soft and unresisting schists of Lias and 

 earlier formations. They then enter a region of folded Jurassic 

 and Cretaceous strata, where they cut through a plateau of synclinal 

 character, the mean height of which is 6260 feet, while the river 

 beds reach no more than some 2000 feet. Still further down, below 

 Gimri, the four streams unite at a height of about 1100 feet, and 

 then, as the Sulak, pass through a range of mountains which reach 

 more than 6500 feet in mean elevation. 



For these phenomena I can find no explanation so well supported 

 by the data at hand as the hypothesis that the bed of the Koissu 

 rivers and the Sulak originally lay at so high a level that the water 

 flowed over the great range of mountains without cutting it, and 

 that the transverse valley which now exists was subsequently eroded 

 to its present depth, its erosion keeping pace with the general 

 denudation of the valleys which lay behind. I came to this con- 

 clusion when I was travelling in Daghestan in 1888, though I had 

 then given no special attention to the general question of transverse 

 valleys. After my return I was induced to study the subject more 

 closely, and I then discovered that the same or similar hypotheses 

 had been advanced long before. 



If we apply this theory to the special case which we are con- 

 sidering, we shall soon see how these rivers in their upper channels, 

 where they flow over soft and easily destructible beds of schist, 

 have managed to hollow out the wide open valleys which there 

 occur, while in the solid and compact limestones and dolomites of 

 the lower part of Inner Daghestan, they have only been able to 

 produce deep narrow cuttings with precipitous walls. General 

 denudation has reduced the level of the upper schist region, till 

 its elevation has become less than that of the limestone and dolomite 

 plateaux and that of the great mountain ridge, all of which once 

 lay below it. The same explanation which serves for the Sulak 

 gorge below Gimri will apply to the narrow defiles at Gunib and 

 Salti in the lower course of the Kara-Koissu, and they may also 

 be regarded as true transverse valleys. 



But the whole of the valley system will be still clearer to us if we 

 consider its original history and gradual development. 



It was probably at the beginning of the Tertiary period that Inner 

 Daghestan rose above the sea and became dry land. I have already 

 drawn attention to the important fact that there are no Tertiary 

 formations in Inner Daghestan, for which reason we must suppose 

 that this region already lay above the sea, whereas Outer Daghestan 

 consists mainly of formations of this period. On the other hand 

 we find in the lower part of Inner Daghestan among the Jurassic 

 folds remains of Gault and Aptian (Middle and Lower Greensands) 

 which show that the sea of the Cretaceous period covered at least 

 a part of this region. This is also proved by the enormous Senonian 

 deposits which form the dividing ridge between Outer and Inner 

 Daghestan and show a thickness of more than 3000 feet. At the 

 beginning of the Tertiary period, however, it was probably this 



