ON THE PROBABLE ORIGIN AND AGE OF TEE SUN. 



Off 



to believe that the present is materially different 

 from the past. No doubt the average rainfall 

 during some past epochs might have been greater 

 than at present, but there is just as little reason 

 to doubt that during other epochs it might have 

 been less than now. We may, therefore, con- 

 clude that about one foot of rock removed from 

 the general surface of the country in 6,000 years 

 may be regarded as not very far from the average 

 rate of denudation during past ages. 



But some of the cases we have given of great 

 denudation refer to comparatively small areas, 

 and others to beds which form anticlinal axes, 

 and which, as is well known, denude more rapid- 

 ly than either synclinal or horizontal beds. We 

 shall, therefore — to prevent the possibility of 

 over-estimating the length of time necessary to 

 effect the required amount of denudation — as- 

 sume the rate to have been double the above, or 

 equal to one foot in 3,000 years. 



To lower the country one mile by denudation 

 would therefore require, according to the above 

 rate, about 15,000,000 years ; but we have seen 

 that a thickness of rock more than equal to that 

 must have been swept away since the Carbonifer- 

 ous period. For, even during the Carboniferous 

 period itself, more than a mile in thickness of 

 strata in many places was removed. Again, there 

 can be no doubt whatever that the amount of 

 rock removed during the Old Red Sandstone 

 period was much greater than one mile ; for we 

 know perfectly well that over large tracts of 

 country nearly a mile in thickness of rock was 

 carried away between the period of the Lower 

 Old Red Sandstone and the Carboniferous epoch. 

 Further, all geological facts go to show that the 

 time represented by the Lower Old Red Sand- 

 stone itself must have been enormous. 



Now, three miles of rock removed since the 

 commencement of the Old Red Sandstone period 

 (which in all probability is an under-estimate) 

 would give us 45,000,000 years. 



Again, going further back, we find the lapse 

 of time represented by the Silurian period to be 

 even more striking than that of the Old Red 

 Sandstone. The unconformities in the Silurian 

 series indicate that many thousands of feet of 

 these strata were denuded before overlying mem- 

 bers of the same great formations were depos- 

 ited. And again, this immense formation was 

 formed in the ocean by the slow denudation of 

 preexisting Cambrian continents, just as these 

 had been built up out of the ruins of the still 

 prior Laurentian land. And even here we do not 

 reach the end of the series, for the very Lauren- 



tians themselves resulted from the denudation, 

 not of the primary rocks of the globe, but of pre- 

 viously-existing sedimentary and probably igne- 

 ous rocks, of which, perhaps, no recognizable 

 portion now remains. 



Few familiar with the facts of geology will 

 consider it too much to assume that the time 

 which had elapsed prior to the Old Red Sand- 

 stone was equal to the time which has elapsed 

 since that period. But if we make this assump- 

 tion, this will give us at least 90,000,000 years as 

 the age of the stratified rocks. 



That the foregoing is not an over-estimate of 

 the probable amount of rock removed by sub- 

 aerial denudation during past geological ages 

 will appear further evident from the following 

 considerations: The mountain -ridges of our 

 globe, in most cases, as is well known, have been 

 formed by sub-aerial denudation ; they have been 

 carved out of the solid block. They stand 2,000, 

 4,000, or 5,000 feet high, as the case may be, 

 simply because 2,000, 4,000, or 5,000 feet of rock 

 have been denuded from the surrounding country. 

 The mountains are high simply because the coun- 

 try has been lowered. But it must be observed 

 that the height which the mountains reach above 

 the surrounding country does not measure the 

 full extent to which the country has been lowered 

 by denudation, because the mountains themselves 

 have also been lowered. The height of the moun- 

 tains represents merely the extent to which the 

 country has been lowered. In the formation of 

 a mountain by denudation, say 3,000 feet in height, 

 probably more than G,000 feet of strata may have 

 been removed from the surrounding country. 

 The very fact of a mountain standing above the 

 surrounding country exposes it the more to denu- 

 dation, and it is certainly not an exaggerated as- 

 sumption to suppose that while the general sur- 

 face of the country was being lowered 6,000 feet 

 by denudation, the mountain itself was at least 

 lowered by 3,000 feet. 



The very common existence of mountains 

 2,000 or 3,000 feet in height, formed by sub- 

 aerial denudation, proves that at least one mile 

 must have been worn off the general surface of 

 the country. It does not, of course, follow that 

 the general surface ever stood at an elevation of 

 one mile above the seadevel, since denudation 

 would take place as the land gradually rose. We 

 know that the land was once under the sea, for it 

 was there that it was formed. It is built up out 

 of the materials resulting from the carving out 

 into hill and dale, through countless ages, of a 

 previously-existing land, just as this latter had 



