Joty—An Lstimate of the Geological Age of the arth. 61 
The mean of all these affords 5:08 per cent. of alkalies—the potash in each case 
exceeding the soda. 
In this same table of Rosenbusch’s we find a Devonian Roofing-slate, Erdstollen, 
with 3:04 per cent. of alkalies. ‘Three other Devonian Slates, not named as 
Roofing-slates (Nos. 2, 38, and 4), show a mean of 3°54 per cent. 
In the Culm we find a Roofing-slate having 3:22 per cent. ; another Culm 
Roofing-slate 5:00 per cent.; and an Upper Culm grey Clay Slate of the 
Fichtelgebirge, 2°99 per cent. | 
If we compare with these ancient sediments those now being deposited, we 
obtain the following figures :—Bischof records 1:47 per cent. of alkalies in the 
suspended matter carried down by the Rhine near Bonn.* Although this is fast 
moving water, the general analyses otherwise closely resembles Roofing-slates and 
Clay Slates, as Bischof points out. The mud of the Nile, near Cairo, affords 1:96 
per cent.f Merrill gives two analyses of fine muds washed by the sea into harbours 
and bays on the coast of North Carolina. They are fine, dark-coloured muds, 
brought down by the rivers and mixed with some decaying animal and vegetable 
matter. These contained 1:97 per cent. and 2°17 per cent. of alkalies; or, 
deducting all organic matter and water (which are temporary constituents), 
these numbers rise to 2°37 and 2°39 per cent. The mean given by these four 
modern silts and muds 1s 2°05 per cent. of alkalies. 
Without further investigation the facts recorded can only be advanced as 
suggestive. 
1X.—The Solvent Denudation of the Ocean. 
This subject, of course, closely concerns the matter discussed in this Paper. 
To assume that no solvent action was exerted by sea-water upon the coasts, and 
the detrital remains continually being poured into it, would, of course, be erroneous. 
We can only hope, in the present state of our knowledge, to find some clue as to 
the magnitude of the time allowance justified by marine solvent denudation. 
In the first place, it is to be noticed that this denudation must be progressing 
chiefly along the immediate coast-lines of the land areas. We can readily arrive 
at a rough estimate of the area involved. Measurement on a terrestrial globe 
shows that the coast-lines of the continents and principal islands amounts to 
132 x 10° miles.— Much of this is rock-bound. Along the rock-bound shores the 
rate of denudation, apart from attrition, is probably extremely slow. Soils cannot 
here accumulate. Particles removed by attrition are carried out and quickly laid 
* «Chemical and Physical Geology,” p. 128. | Loe. cit., p. 138. 
{ Croll, allowing for bays and inlets and the smaller islands, estimates the coast line at 116 x 10% miles. 
Wallace takes 100 x 10° miles, See ‘‘ Island Life,” p. 221. 
