C, Davison — Lead Age of the Stratified Rocks. 349 



Let V be the number of cubic feet of sediment brought down by 

 a river every year, and A the area of its delta in square feet. If the 

 whole of the sediment were spread over the present delta, so as to 

 keep its surface close to the surface of the sea without enlarging the 

 area of the delta, the average rate of subsidence would be F-;- ^ feet 

 per year. If, however, as is generally the case, the sediment is 

 spread over a larger area, increasing the delta laterally, then the 

 average rate of subsidence must be less than V -^ A feet per year. 

 Thus, the volume of sediment brought doicii hy a river in a year, 

 divided by the area of the delta, gives a superior limit to the average 

 rate of annual subsidence in the region of the delta. 



As the area of the delta increases, this limit decreases, continually 

 approaching the true mean rate of subsidence. The true mean rate 

 would also of course be known, if we could ascertain the amount of 

 surplus sediment used for increasing the area of the delta, and there- 

 fore the amount left for keeping the delta-surface in shallow water. 



It should be noticed that the mean rate of subsidence is indepen- 

 dent of the recurrence of glacial periods and of all changes that 

 may take place in the intensity of the denuding forces. True, in 

 estimating its superior limit, we have left out of account the materials 

 derived from littoral erosion and from, some volcanic eruptions that 

 may be swept by currents into the delta-area ; but we cannot err 

 greatly in neglecting them. 



As an example, let us take the case of the Mississippi. Dr. A. 

 Geikie states that the area of its delta is 12,300 square miles, or 

 342,204,320,000 square feet; and that the volume of sediment 

 brought down by the river every year is 7,459,267,200 cubic feet.'^ 

 Hence, the average rate of subsidence throughout the area of the 

 delta cannot be greater than ve of a foot per year, or, more exactly, 

 2*18 feet per century. 



Now, the mean rate of subsidence is not uniform throughout the 

 whole period of formation of a fold, for it is evidently zero at the 

 very commencement and also at the epoch when subsidence changes 

 to elevation, and there may be pauses in the process. But, in 

 different parts of the earth, there must be folds in different stages of 

 formation ; and the average of the mean rates of subsidence of all 

 known areas of sedimentation will give the true mean rate of sub- 

 sidence for the present time. Not having the necessary data, how- 

 ever, for river- deltas other than that of the Mississippi, we may 

 assume the highest value of the mean rate of subsidence at the 

 present time to be Ye of a foot per year, which, it is worth noticing, 

 is of the same order of magnitude as the estimated rate of elevation 

 of the north coast of Scandinavia. 



3. The mean rate of subsidence over the whole earth has not, 

 however, been uniform throughout geological time ; and it remains 

 for us to determine, as nearly as we can, the law according to which 

 it changes. This, I believe, it is possible to do, if we accept the 

 hypothesis of a solid globe cooling from a high initial temperature. 

 In a paper lately read before the Eoyal Society, I have shown that 

 1 Text-book of Geology, pp. 389, 444. 



