Vice-President’s Address. 267 
origin over a given area cannot much exceed the rate at 
which that material was wasted by subaerial agencies from 
an equal area of land. But when it comes to particulars, 
we are confronted with so many difficulties that it seems 
impossible, with the knowledge we can at present command, 
to make any statement regarding the rate of formation of any 
kind of terrigenous deposit, which can be regarded as much 
more than a speculation. Even if, in the case of any given 
river-discharge, we knew the rate at which the various sea 
currents were flowing, and we were sure of their direction in 
all cases, we should still have to take into account the fact 
that all delta-areas are areas of subsidence, and, further, that 
both the rate of subsidence and the rate at which the down- 
ward phase of terrestrial undulation which it represents is 
progressing, are factors of the utmost importance in working 
out a question of the kind before us. It is only, it seems to 
me, in the case of marine deposits of organico-chemical 
origin that we have any data which appear to be at all 
trustworthy in this respect. These, at any rate, are not now 
so much affected by local causes, and probably have not 
been so in the past. Moreover, the influence of diffusion 
throughout the whole oceanic area tends rapidly to equalise 
the percentage of mineral substances held in solution. 
Hence, for example, if a river draining a limestone district 
where there is a heavy rainfall, carries into the ocean at 
one part exceptionally large quantities of lime-salts, that 
quantity is soon equalised throughout the whole area of the 
ocean. In like manner, if, as in the case of the Pacific and 
Indian Oceans, the growth of coral reefs locally drains the 
surrounding water of its lime-salts at an exceptionally rapid 
rate, the balance is soon made good by supplies carried into 
the ocean from points hundreds or may be thousands of 
miles distant. Hence, ignoring coral-reefs as a form of lime- 
stone which is confined to the Post-Miocene Periods of the 
Earth’s history, marine limestones appear to afford us the 
most satisfactory data upon which we can base our con- 
clusions ; as such, in the remainder of this address, I propose 
to employ them. 
Mode of Formation and Rate of Growth of Marine Lime- 
