Taking the quantity of sediment discharged into the sea annually by the Mississippi river, as determined by Messrs. Brown and Dickson,[23] I found that it amounted to one foot off the face of the country in 1,388 years, and that, at this rate of denudation, our continents, even if they had an elevation of 1,000 feet, would not remain above sea-level over 1,500,000 years.[24] This was an exaggerated estimate of the quantity of sediment, for I shortly afterwards found that far more accurate determinations were made by Messrs. Humphreys and Abbot,[25] who were employed by the United States Government to report upon the physics and hydraulics of the Mississippi. Messrs. Brown and Dickson had estimated the quantity of sediment at 28,188,083,892 cubic feet, whereas Messrs. Humphreys and Abbot found it to be only 6,724,000,000 cubic feet, or less than one-fourth that amount. This gives one foot in 6,000 years as the rate of denudation. [Footnote 23: _Proceedings of the American Association for the Advancement of Science_ for 1848.] [Footnote 24: _Philosophical Magazine_, February 1867. I was not aware at this time that Mr. Alfred Tylor had previously applied the same method to determine an entirely different point, viz.: how much the sea-level is being raised by the sediment deposited on the sea-bottom. Mr. Tylor’s paper, entitled “On Changes of the Sea-Level effected by existing Physical Causes during stated Periods of Time,” appeared in the _Phil. Mag._ for April 1853. Mr. Tylor came to the conclusion that the sea-level was being raised, from this cause, about 3 inches in 10,000 years.] [Footnote 25: _Report upon the Physics and Hydraulics of the Mississippi._] At this time Dr. Archibald Geikie took up the question and went into the consideration of the subject in a most thorough manner; and it is mainly through the instrumentality of his writings on the matter[26] that the method under consideration has gained such wide-spread acceptance among geologists. After an examination of nearly all that is known regarding the amount of sediment carried down by rivers, he drew up the following table, showing the number of years required by seven rivers to remove one foot of rock from the general surface of their basins. Danube 6,846 years Mississippi 6,000 „ Nith 4,723 „ Ganges 2,358 „ Rhone 1,528 „ Hoang-Ho 1,464 „ Po 729 „ ----- ----- Mean 3,378 years