250 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Feb. 1, 



the mountains at two distant points, at opposite ends of the chain. 

 4 th. The constancy maintained for great distances along the length 

 of the chain, both in the geological structure, and in the elevations 

 to which the mountains rise ; which last character is particularly 

 shown by the almost perfect horizontality of some of the most elevated 

 stratified deposits, by the extremely small slope of the main drainage- 

 channels of the Tibetan table-land, and by the general similarity of 

 the altitudes even along the most rugged ridges. 



After allusion to the question of the reality of upheavements, as 

 opposed to the view which attributes elevations on the earth's surface 

 to subsidences of the lower parts, and ha\dng stated reasons for be- 

 lieving that in the present instance the elevation was real, a review 

 was taken of the igneous rocks which might appear to have been 

 connected with the upheavements of these mountains, and the conclu- 

 sion was arrived at, that none of them were specially related to the 

 last great movement that had taken place. 



Hence it was inferred that the agent of elevation was probably a 

 development of elastic vapours, as has been considered by Mr. Hop- 

 kins to be likely in a general point of view. The probability of 

 this in the present instance was shovra to be upheld by the extreme 

 regularity of the upheavement, when regarded as a whole, which 

 seems to indicate a flvdd body as the medium through which the 

 forces have been applied to the superincumbent masses ; while this 

 regularity also shows the comparatively great thickness of the part 

 of the earth's crust that has been raised, as compared to the heights 

 of individual ridges or peaks above the general level. 



Assuming the general unity of the upheavement of the whole area 

 as evident from the consideration of the form of the section of 

 the mountains, the results of such an upheavement were then 

 examined by help of the principles laid down by Mr. Hopkins. 



The first of these results was stated to be a main system of longi- 

 tudinal fissures, parallel to the major axis of the elevated area ; with a 

 secondary system perpendicular to the former. As the manner in 

 which these fissures would open, and all the subsequent effects of the 

 upheavement, must depend in great measure on the state of the in- 

 terior of the earth, of which we can know nothing directly, we must 

 be guided in our suppositions on this point by the phenomena 

 actually observed in connexion with the ruptures at the surface ; 

 and, in the present case, we seem to be able to account for many of 

 the peculiarities of structure already noted, by conceiving that the 

 state of the upheaved mass, at the moment of rupture, was similar to, 

 but just the reverse of, that of a beam supported and held down at 

 the ends, and loaded at the centre. 



On this hypothesis we should expect to find the chief longitudinal 

 ruptures in the centre of the area, with others important, but in a 

 less degree, along the margins, while there would be two portions 

 intermediate between the axis and the edges, where the tendency to 

 rupture would be a minimum. Further, as the tendency to trans- 

 verse rupture is proportional to the amount of the elevation, it would 

 have a minimum along the edges of the area ; at the same time, as 



