Correspondence — Mr. Alfred Tylor. 525 



and a perpendicular to tlie horizontal line, which measures the 

 " obliquity of trend," caused, as it will be observed, by the shadow- 

 receiving plane being inclined to the horizon. If the plane be 

 ■vertical, there will be no such obliquity. 



0. FlSHEE. 



EATE OF DENUDATION OF THE LAND BY RIVERS. 



Sir, — In a paper published in April, 1853, in the Phil. Mag. 

 I have calculated the mean denudation of the land by rivers and 

 the sea to be equal to three feet in 10,000 years, taken over the 

 whole surface of the land, I further corrected this in 1875, 

 Appendix Geol. Mag. p. 433, et seq., by taking into account the 

 quantity of material not in suspension, viz. sand, etc., pushed out 

 to sea by rivers. I find this equal to twice as much denudation as 

 the material carried out to sea in suspension indicated by a new 

 method. 



I omitted this point in 1853, and Mr. James Croll, who followed 

 my method of calculations, has always omitted it also. 



This denudation would be nearly ten feet in 10,000 years, or one 

 foot in 1000 years, with the present rainfall. But in the Pluvial 

 period the rainfall would be 300 inches, or about ten times as great 

 as the present. Belgrand afterwards suggested a twenty-fold rainfall. 



By my formula of the increase of velocity of water at the same 

 slope, according to increase of quantity, I found the velocity of 

 streams would be enormously increased in the Pluvial period, 

 particularly in the rainy seasons, as the quantity flowing in rivers 

 would be enormous. If the quantity of rain increased eight times, 

 the velocity of the stream would be double ; but if the rainfall was 

 very unequal, the mean velocity for the year might be much in 

 excess, say three times the present velocity. In the Pluvial period, 

 if the mean velocity may have been three times the present mean 

 velocity of streams and rivers, Hopkins has shown the power of 

 moving material increases in an enormous ratio with the velocity. 

 If the moving force for removing strata is as the fifth power, 

 and the velocity 3 times, then as 3 ^ equals 729, the mean denuda- 

 tion in the Pluvial period would be 729 times the present. This 

 would be equal to a mean removal of 9 inches in the year off the 

 land, and a mean deposit of 3 inches in the sea, raising the sea-level 

 to that extent. 



The deltas of all great modern rivers are formed of thin stratified 

 beds containing land plants, and always recent fresh-water shells 

 that can only live in shallow fresh-water, or shells or animals living 

 in estuaries. The surface of these modern deltas is always near the 

 level of the sea at the shore-line, and must have always been so 

 during their formation, as they are all shallow-water deposits. As 

 the sea-level rose during the supposed period of 729 years, the 

 deposits must all that time have exactly kept up with the elevation 

 in order to keep marine deposits away. The depth of these delta 

 deposits is from 500 to 1000 feet, as ascertained by borings. 



In 3000 years, with a mean denudation of laud of 9 inches a year, 



