Trof. Milne — Across Europe and Asia. 



31 



This section runs from S.E. to N.W. across the valley of the Eiver 

 Irkut, from the Syen Mountains to the Tunkusian Alps. These 

 latter are made up of limestones and pyroxenic rocks which are 

 much metamorphosed. All that is known about them at present is, 



Syen 

 Mountains 



S.E. 



A, A, = Alluvium. L,L, = Lava. 

 Diagram section of the Irkut Valley (after M. Tchersky). 



that they are older than the Jura sandstones, beneath which they are 

 seen to pass. In the valley between these two ranges of mountains 

 there are beds of lava and of alluvium. This, like that on the road 

 to Kiachta, is basaltic in character. I often inquired about felspathic 

 lavas, but I never heard of them. The colour of this lava is black, 

 or some shade of black or bluish black, and occasionally grey. It 

 is generally compact. When it is scoriaceous, it sometimes has a 

 reddish-grey colour. Many of the specimens which I saw con- 

 tained much olivine. In some places, as near Tunka, there are 

 small hills of lava. These were at first thought to be old craters, 

 but are now only supposed to mark the places where the lava may 

 have welled up through fissures. The beds of lava are not the 

 result of one fiow, but of several. This may be seen at several 

 points where there is evidence of one bed of lava having flowed, 

 become cool and scoriaceous in its surface, before a bed which 

 may be seen lying upon it ever issued from the crater. In some 

 places as many as six beds may be seen lying one upon the other, 

 each compact at its base, but gradually merging into a scoriaceous 

 character above. At one locality M. Tchersky described to me 

 instances of the peculiar concentric structure which the lava had 

 assumed. 



From the evidence we have, the outburst of the Tunkusian 

 volcanos must have extended over a long period of time, apparently 

 having commenced before the excavation of the Tunka valley, which 

 they partly filled with their products. The next step was the 

 wearing away of these lavas into hollows, and into these an 

 alluvium, probably identical with that which now covers a great 

 portion of Siberia, was deposited. That this deposition was general 

 in its nature is evident from the corresponding position of similar 

 beds in neighbouring depressions. At the base of the alluvium 

 there are rounded pebbles of basalt derived from the rocks beneath. 

 In these deposits Mammalian remains such as of the Mammoth and 

 Ehinoceros have been found. Some of these beds are found as 

 much as 1157 feet above the village of Tunka, which is itself at 



