Vice-President’s Address. 285 
of the Carboniferous Rocks of that part. In the area referred 
to, the Upper Carboniferous Rocks (including under that 
term, for the present purpose, all the rocks from the 
highest remaining Coal-Measures to the top bed of the 
Carboniferous Limestone) are estimated to be fully 12,000 
feet in thickness. These consist of alternate beds of sand- 
stone, shale, fireclay, and coal. In dealing with these strata, 
we are forced to confess that we have not, and probably may 
not obtain yet for many years, any reliable estimate of the 
several rates at which delta deposits are at present being 
laiddown. The most that can be done in the present state of 
our knowledge is to form an estimate from the “Challenger ” 
maps of the ratio between the superficial area occupied by 
the purely terrigenous deposits now in process of formation, 
to compare this area with the area of the land whence 
drainage finds its way seaward, and then on this basis com- 
pute the proportion between the area of deposit and the 
proportion of material mechanically carried seaward by rivers. 
In the case of lime-salts, a question of this kind is simplified 
by the fact that, owing to rapid diffusion of matter carried in 
solution, especially lime-salts, an excess of these poured into 
the ocean by the rivers of one area is rapidly equalised by 
diffusion over the entire ocean area. An average rate, com- 
puted from observation made upon several rivers in different 
parts of the globe, therefore holds good for the whole. With 
terrigenous deposits the case is widely different. No diffu- 
sion worth considering appears to take place. On the 
contrary, each river has its own rate of deposit, which again, 
unlike the rate of diffusion of solutions, may be very different 
now from what it was at the same spot in former times, 
when different conditions obtained. Further, the matter is 
complicated by oscillations of level of the land, and by the 
fact that, unlike calcareous deposits of marine origin, marine 
terrigenous deposits tend to be more or less lenticular in form. 
A rough estimate, however, based upon a comparison of 
ocean-drainage areas to the area of the terrigenous deposits 
in each case (the volume being yet unknown), gives us the 
following :—In the Indo-Chinese area the superficial area of 
the terrigenous deposits is to the drainage area about as 
