BY H. I. JENSEN. 543 



the great basin of Mesozoic sedimentation which extends across 

 the Tibet and Himalayan region, and to the south of an Archaean 

 massive which escaped Mesozoic sedimentation. Portions of the 

 Tien-Chan area were subjected to sedimentation by transgressions 

 of the sea in Triassic, Jurassic and Cretaceous times. Folding, 

 according to Suess, is towards the south. The volcanic rocks are 

 considered to be of Tertiary age. 



Alkaline rocks from Eastern Siberia have been described by 

 H. S. Washington in the American Journal of Science, Vol.xiii. 



Eurasia being made up of a number of portions of widely 

 different origin, and containing parallel mountain-ranges due to 

 step-faulting as in North-east Asia, mountain-knots having the 

 nature of horsts, like those of Bohemia and the Harz; mountains 

 folded towards the south like the Himalayas, and those folded 

 towards the north like the Alps, is so complex that it is often 

 difficult to interpret the relationship of its alkaline rocks to earth 

 structure. 



A few generalisations may nevertheless be made out. 



(1) It is noticeable that the alkaline rocks may be situated on 

 horsts, as in Central France and Bohemia, that is on the fractured 

 relics of very ancient continents. The foyaitic rocks of Christiania 

 occupy a subsidence-area on the southern edge of the Scandinavian 

 massive, those of Hogbau on a subsided portion of the Bohemian 

 massive. Those of Armenia line fractures which intersect this 

 northern relic of the Indo- African continent. The foyaitic rocks 

 of Kola lie on the border of the Scandinavian massive, and a por- 

 tion of the same continent which in early Tertiary times was 

 severed by means of fractures and subsided, thus cutting off 

 Spitzbergen from Scandinavia. To the north-east of Kola lay an 

 area of Mesozoic sedimentation. The Miask alkaline rocks lie on 

 the eastern flank of the Russian massive, where it borders on the 

 basin which in Mesozoic times extended from the Kola Sea and 

 Arctic Ocean down to the Aral and Caspian Seas. 



(2) Alkaline rocks are found on the borders of great Mesozoic 

 sedimentary basins, and sometimes on areas over which such seas 

 have transgressed for brief periods, but they are not found in the 



