346 Archdeacon Pratt on the Thickness of 



feet above the sea : at this poiut the mountains begin decidedly 

 to rise, there being only the low Sewalik Range to the south. 

 The diagram represents a meridional section of the mountain 

 mass through Dehra. A B is about 140 miles, or 2 degrees ; B C 

 is about 230 miles, or 3^ degrees : take C D equal to A B. The 

 southern mountain slope A B rises from A gradually to a height 

 of 15,000 feet at b, the intervening space abounding in peaks of 

 20,000 feet and more. From Z» to c is the vast plateau, vrhich 

 Colonel Strachey places at an average height of 15,000 feet 

 above the sea. The northern slope from c onwards is not so 

 well known ; and I do not enter upon any description of it, because 

 it is not required for my purpose. 



Draw chord A D, and let amm' nhe a vertical line through a, 

 the middle point of the plateau, and cutting the arc A D in m, 

 the chord A D in m', and meeting the interior surface of the crust 

 in n. Let A ?• be the thickness of the crust at A ; es the thick- 

 ness at some other point e on the north. Suppose the line rns 

 is the form of the section of the inner surface of the crust, as 

 Abe e in that of the outer surface. 



2. Owing to the downward pressure of the superincumbent 

 mass lying on A B C D, the tendency is for the point a to sink, 

 and the point a n to open at n ; the points i- and s to rise, being 

 forced up by the lava under them, which communicates the excess 

 of pressure which the redundant mass of the plateau produces in 

 the lava below it at n and thereabouts. This tendency in r and 

 s to rise would cause the joints A r and es io open at A and e, 

 were it not for the adhesion of the sides of each joint. 



3. It may be thought that, as the points r and s cannot move 

 horizontally, the triangle r as (supposing the lines joined) must 

 remain invariable, and therefore a cannot sink. But the answer 

 is plain. If the adhesion at the joints Ar, An, es be not suffi- 

 cient to resist the tendency to open, the moment of the whole 

 weight of the redundant matter will be thrown upon the three 

 points a, r, s ; and even if we assume that the materials at these 

 points will not yield by crushing, it is evident they must give 

 way by the angles breaking off, in consequence of the enormous 

 cross strain which will come upon them, and which they cannot 

 resist, owing to the cracks and fissures and general incoherence 

 of the materials of which the crust is made. 



4. Our attention now is to be directed to the conditions of 

 equilibrium of the mass A?-waA, supposing that the crust is 

 sufficiently thin for the principles of hydrostatics to have effect. 



The continent of Hindostan to the south of A may be regarded 

 as one vast plain stretching down to the sea, because the actual 

 variations of surface are very trifling compared with the parts I 

 am considering. The portion of the crust of which that conti- 



