TRANS-HIMALAYA AND TIBET 47 



arcs ; and the Trans-Himalayan arcs would have had a corre- 

 spondingly restricted area in which to develop. It will also be 

 noticed that the orientation of the Trans-Himalayan arcs has 

 more in common with that of the Great Himalaya on the south 

 side of the Brahmaputran trough than with the latitudinal 

 alignment of the ranges of Tibet, that the convexity of the 

 arcs decreases from north to south, and that structurally Trans- 

 Himalaya must be regarded as part of the Himalayan system. 

 Its mountain-folds must also have been formed at a date anterior 

 to those of the Tibetan plateau. 



In conclusion it would seem to be something more than a 

 mere chance coincidence that both in the Armenian and in the 

 Tibetan plateau we should find so close a similarity of structure. 

 In both cases the plateau is delimited from its southern border- 

 ranges by a fracture cutting off abruptly the structural lines of 

 the more ancient rocks of these border-ranges, so that they form 

 an acute angle with the nearly latitudinal orographic features 

 of the plateau. In both cases a depression, marked by a zone 

 of lakes or lake-deposits, occurs along this fracture. In both 

 cases the border-ranges have been fractured into blocks, which 

 either have been uptilted or else have sunk down to form deep 

 rift-valleys, and in the act of sinking have squeezed up molten 

 lava to issue from volcanic vents along the lines of fracture. In 

 both Armenia and Tibet the outer zone of their border-ranges 

 is occupied by Tertiary strata, which have been thrown into 

 folds and are separated in each case from the ancient tablelands 

 of Arabia and India by the broad and deep alluvial depressions 

 of Mesopotamia and the Ganges respectively. 



All these points of similarity in structure and relative position 

 appear to be sufficiently striking in two members of the con- 

 tinuous plateau-belt stretching from the confines of China to the 

 iEgean Sea to permit of being homologised in a comparative 

 table. The close parallelism of all the corresponding oro- 

 graphical units is in this manner succinctly emphasised and 

 the table printed on the opposite page may serve to bring 

 out the essential unity of construction underlying the formation 

 of each of these lofty plateaux. 



