286 Proceedings of the Royal Physical Society. 
two-thirds to one. In the Eurasian Atlantic drainage area 
the extent of the terrigenous deposits is to that of the 
oceanic drainage area also as two-thirds to one. On the 
other hand, the North American Atlantic drainage area gives 
rise to a much smaller fringe of terrigenous deposits, which 
form certainly less than half the area from which their 
materials are derived. In the South American Atlantic 
drainage area the terrigenous deposits occupy not more than 
one-third of the area of the land. In this case, as in the 
case of Africa, the proportion appears to be smaller than it 
really is, because the terrigenous deposits fall rapidly into 
deep water, and their superficial extent is therefore less in 
proportion to the thickness than it might otherwise be. As 
pointed out above, we can have no definite estimates of the 
rate of formation of delta deposits (not, at any rate, in terms 
of feet per century) until we know the amount and nature of 
the material transported, the rate of subsidence, and the 
strength and direction of the marine currents outside the 
delta. What we require to know is not so much the area 
alone which is formed in a given delta in a given time, as the 
thackness which is deposited in that time. 
Dr John Murray’s figures and maps give us as the total 
area of ocean drainage 44,211,050 square miles, and 
27,899,300 square miles as the approximate area of terri- 
genous deposits. This is in the proportion of rather more 
than one and a half of drainage area, or area of denudation, to 
one of the ocean area over which terrigenous deposits are 
being laid down. This would seem at first sight to imply 
that the deposition of strata goes on half as fast again as 
denudation, and that a foot and a half of strata is laid down 
upon the sea-bed in the same time that is required to strip one 
foot of rock from the general surface of the land. That this is 
far from being the case will be evident when we consider 
that a large part of the sand and loam transported seawards 
from the land finds a resting-place close to the land itself. 
It is readily conceivable that loam (which is a mixture of 
fine sand with a small percentage of clay) should be laid 
down at the rate, say, of 1 foot in 2000 years. The finer clay, 
however, drifts far and wide before it subsides, and we should 
