472 Denudation of Earth Measured by Improved Method. 



the Delta, which I omitted, and estimating the proportion of quartz rock to 

 clay slate as 2 to I, in the district under denudation, I now calculate the 

 average rate of denudation in the Mississippi area (which is supposed to be the 

 average of the world) at I foot in 2,000 years, of which 8 inches is siliceous 

 matter, and the other silex and clay in the proportion they are found in clay 

 slate. The usual proportion of silica and alumina in the solid matter suspended 

 in river water is that in clay slate, according to Bischoff : that is, silica 65*1 

 and alumina i6"38 per cent, in clay slate. In the Rhine water Bischoff found 

 the sediment, or matter in suspension, composed of 6377 silica and IS'S4 

 alumina. On the contrary, in the Permian beds he found silica 97 and 

 alumina 2 per cent. 



The proportion in the rocks of the globe of silica and alumina gives 

 a means of calculating the rate at which the whole surface of the earth 

 is lowered. The wetted perimeter at Fort St. Philip, Mississippi, is 2,576 

 feet. If the sand on the river bottom and channel moves out to sea 

 1 foot deep at 10 feet per minute, twice as much material will be • con- 

 veyed beyond the mouth of the river along the bottom as is carried away 

 in suspension. Colonel Tremenheere, in 1866, made a series of observations 

 on the movement of water coast-ways on the coast of India. He found a con- 

 stant current in one direction at a mile an hour, taking floats from the Indus' 

 mouth right into the harbour of Kurrachee. Mr. Croll has entirely omitted 

 from his calculations the enormous mass of sand pushed out or carried out to 

 sea by rivers, in his late estimate of denudation in climate and time. He has 

 also mistaken the figures, I printed first, of the coast line of continents, for the 

 coast line of the whole world. In Phil. Mag. 1853, I gave Fig. 3, page 263, 

 the proportion of the coast line of the whole world, and made an estimate of 

 the denudation by the sea, taking the coast line of islands as twice as much as 

 continents. Mr. Croll has varied my figures, and arrived at a conclusion that 

 no one could admit as to denudation. I consider that after a long denudation 

 the coast line would be very much lengthened, and the surface of the land 

 would be as much raised by blown sand and coral banks as it was lowered by 

 other actions. 



4 Note to pages 442 and 449. — Dubuat has been misunderstood on this subject 

 by Mr. Croll. Dubuat limited his remarks on the relation of motion to slope to 

 the case of a little canal, and never intended his remarks to apply to the relative 

 motion and slope of the ocean. Mr. Croll's argument on this point with Dr. 

 Carpenter is unfortunately based upon an incorrect reading, of Dubuat, and 

 cannot be maintained. 



s Note to page 456. — The Niagara limestone is 80 feet thick, lying on the Niagara 

 shale, also 80 feet thick, according to Dana, where the Niagara waterfall is 

 60 feet high. The covering of the shale by falling blocks of limestone, is not 

 noticed by him, although it must be, I should think, an important feature. 



6 Note to page 457. — The gradient from the surface of the Weald clay once 

 uponCrowborough Beacon (then 1,700 feet high), in the direction of Sevenoaks 

 Weald, near Hildenborough (400 feet high), close to the south end of the 

 Sevenoaks tunnel, was a comparatively flat surface, about 80 feet per mile ; that 

 is, the fall, from 1,700 feet to 400, or 1,300 feet in 17 miles, is 80 feet per 

 mile ; while in the gorge at Dorking the Weald clay falls 800 feet in a mile. 

 In one case there was a watershed and a small denudation, in the others a 

 river and great removals, owing to the difference in dip favouring denudation. 

 INoteto Fig. 35, page 462. — This wood-cut, Fig 35, and accompanying expla- 

 nation, was set up in type by Mr. Anstin, of Hertford, in 1872 for the Geological 

 Magazine. My paper on denudation, with allusion to the Weald, was being 

 published, but the end of that paper was cut off for want of space, including the 

 illustration. I reprint this on page 473 from the proof of 1 872 without alteration. 



