554 



THE POPULAR SCIENCE MONTHLY.— SUPPLEMENT. 



must consequently have been carved out of the 

 solid rock between the period of the Old Red 

 Sandstone and the beginning of the Carboniferous 

 age. This affords us some conception of the im- 

 mense lapse of time represented by the Middle 

 and Upper Old Red Sandstone periods. 



Again, in the case of the great fault separat- 

 ing the Silurians of the south of Scotland from 

 the Old Red Sandstone tracts lying to the north, 

 a thickness of the latter strata of probably more 

 than a mile, as we have seen, must have been re- 

 moved from the ground to the south of the fault 

 before the commencement of the Carboniferous 

 period. And, again, in the case of the Lanca- 

 shire coal-fields, to which reference has been 

 made, nearly two miles in thickness of strata had 

 been removed in the interval which elapsed be- 

 tween the Millstone Grit and the Permian pe- 

 riods. 



As we are enabled, from geological evidence, 

 to form some rough estimate of the extent to 

 which the country in various places has been 

 lowered by subaerial denudation during a given 

 epoch, it is evident that we should have a means 

 of arriving at some idea of the length of that 

 epoch, did we know the probable rate at which 

 the denudation took place. If we had a means 

 of forming even the roughest estimate of the 

 probable average rate of subaerial denudation 

 during past ages, we should be enabled thereby 

 to assign approximately an inferior limit to the 

 age of the stratified rocks. We could then tell, 

 at least, whether the amount of subaerial den- 

 udation known to have been effected during 

 past geological ages could have been accom- 

 plished within 20,000,000 years or not, and this 

 is about all with which we are at present con- 

 cerned. And if it can be proved that a period 

 of 20,000,000 years is much too short to account 

 for the amount of denudation known to have 

 taken place, then it is certain that the gravita- 

 tion theory cannot explain the origin and source 

 of the sun's heat. 



A very simple and obvious method of deter- 

 mining the present mean rate of subaerial den- 

 udation was pointed out several years ago, 1 viz., 

 that the rate of denudation must be equal to the 

 rate at which the materials are carried off the 

 land into the sea. But the rate at which the ma- 

 terials are thus abstracted is measured by the 

 rate at which sediment is carried down by our 

 rivers. Consequently, in order to determine the 



1 Philosophical Magazine,, May, 186S ; February, 

 18G7. " Climnte and Time," chap. xx. See also " Trans- 

 actions of the Geological Society of Glasgow," 1 vol. iii. 



present rate of subaerial denudation, we have 

 only to ascertain the quantity of sediment annu- 

 ally carried down by the river-systems. 



Very accurate measurements have been made 

 of the quantity of sediment carried down into the 

 Gulf of Mexico by the river Mississippi, and it is 

 found to amount to 7,474,000,000 cubic feet. 

 The area drained by the river is 1,224,000 square 

 miles. Now, 7,474,000,000 cubic feet removed 

 from 1,224,000 square miles of surface is equal 

 to 4 5^ of a foot off the surface per annum, or 

 one foot in 4,566 years. The specific gravity 

 of the sediment is taken at 1.9, and that of the 

 rock at 2.5 ; consequently the amount removed is 

 equal to one foot of rock in about 6,000 years. For 

 many reasons there are few rivers better adapted 

 for affording us a fair average of the rate of sub- 

 aerial denudation than the Mississippi. In refer- 

 ence to the above I may here quote the words of 

 Sir Charles Lyell : " There seems," he says, " no 

 danger of our overrating the mean rate of waste 

 by selecting the Mississippi as our example, for 

 that river drains a country equal to more than half 

 the Continent of Europe, extends through twenty 

 degrees of latitude, and therefore through regions 

 enjoying a great variety of climate, and some of 

 its tributaries descend from mountains of great 

 height. The Mississippi is also more likely to 

 afford us a fair test of ordinary denudation, be- 

 cause, unlike the St. Lawrence and its tributaries, 

 there are no great lakes in which the fluviatile 

 sediment is thrown down and arrested on its way 

 to the sea." 1 



Rough estimates have been made of the sedi- 

 ment carried down by some eight or ten European 

 rivers ; and, although those estimates cannot be 

 depended upon as being anything like accurate, 

 still they show that it is extremely probable that 

 the European Continent is being denuded at 

 about the same rate as the American. 



I think we may safely assume, without the 

 risk of any great error, that the average rate of 

 subaerial denudation during past geological ages 

 did not differ much from the present. The rate 

 at which a country is lowered by subaerial den- 

 udation is determined J not so much by the char- 

 acter of its rocks as by the sedimentary carrying- 

 power of its river-systems. And this again de- 

 pends mainly upon the amount of rainfall, the 

 slope of the ground, and the character of the soil 

 and vegetation covering the surface of the coun- 

 try. And, in respect of these, we have no reason 



1 " Student's Manual of Geology," p. 91 (second edi- 

 tion). 



2 See "Climate and Time," p. 334. 



