of the Plumb-line in India. 19 
what less than crust density three miles thick where it abuts on 
the foot of the Himalayan slope, and thinning out to nothing 
against the trappean area. Jt will be observed that a pile 

“ Diagram to illustrate the theory of the elevation of the Himalayas 
corresponding to the right-hand half of fig. 26 (copied from ‘ Physics of 
the Earth’s Crust’). Horizontal scale about 60 miles, vertical about 
30 miles to one inch. [This makes the crust much thinner than assumed 
in P. E.C.] 
Massif of the Himalayas. 
Root of the same. 
Earlier marginal deposits compressed and elevated, 
Continuation of the same, depressed and. undisturbed. 
Subsequent deposits overlapping C. 
Sinking of lower surface of crust due to C and D.” * 
aD QS 
of rock three miles thick above the surface of the sphere 
would reach an altitude equal to that of the Tibetan plateau, 
but the Gangetic plains do not rise to as much as a thousand 
feet above the sea-level, and the recent alluvium has to be 
allowed for. The conclusion is that the crust of the earth 
must be depressed 15,000 feet at least into the substratum 
at the foot of the Himalayan slope. We need, therefore, to 
calculate the effect of this arrangement upon the plumb-line. 
According to the present hypothesis, the Himalayas and 
Tibetan plateau are not supported solely by the root im- 
mediately beneath them, but partly by the depressed crust 
beneath the Sivalik rock of the plains. It follows that for 
_ isostacy the root of the plateau and slope will not need to be 
quite so deep ; but since the difference will be small, it will 
be legitimate to consider the attraction of the diminished 
root to bear to the attraction of the hitherto considered root 
the proportion of their masses, 7.e., the proportion of their 
depths below the bottom of the crust, which we must 
calculate. In doing this we may take the height of the 
plains above the sea-level as compensating their presumable 
defect of density from that of the general crust. 
The width of the alluvium, where the meridian of Dehra 
Dun in Mr. Oldham’s geological map crosses its boundary, 
appears to be about 230 miles. This will give for the base 
* * Manual of the Geology of India,’ 2nd ed., Calcutta, 1893, p. 473. 
C2 
