46 SCIENCE PROGRESS 



unless on the assumption that there is something entirely mis- 

 leading in the existing rate of solvent denudation. On the 

 strength of the results of another and entirely different method 

 of approaching the question of the earth's age (which shall be 

 presently referred to), it has been contended that it is too low. 

 It is even asserted that it is from nine to fourteen times too low. 

 We have then to consider whether such an enormous error can 

 enter into the method. The measurements involved cannot be 

 seriously impugned. Corrections for possible errors applied 

 to the quantities entering into this method have been considered 

 by various writers. My own original corrections have been 

 generally confirmed. I think the only point left open for dis- 

 cussion is the principle of uniformitarianism involved in this 

 method and in the methods previously discussed. 



In order to appreciate the force of the evidence for uni- 

 formity in the geological history of the earth, it is, of course, 

 necessary to possess an acquaintance with that history. Some 

 of the most eminent geologists, among whom Lyle and Geikie ' 

 may be mentioned, have upheld the doctrine of uniformity. It 

 must here suffice to dwell upon a few points having special 

 reference to the matter under discussion. 



The mere extent of the land surface does not, within limits, 

 affect the question of the rate of denudation. This arises from 

 the fact that the rain supply is quite insufficient to denude the 

 whole existing land surface. About 30 per cent of it does not, 

 in fact, drain to the ocean. If the continents become invaded 

 by a great transgression of the ocean, this " rainless " area 

 diminishes : and the denuded area advances inwards without 

 diminution. If the ocean recedes from the present strand 

 lines, the " rainless " area advances outwards, but, the rain 

 supply being sensibly constant, no change in the river supply of 

 salts is to be expected. 



Age-long submergence of the entire land, or of any very 

 large proportion of what now exists, is negatived by the con- 

 tinuous sequence of vast areas of sediment in every geologic age 

 from the earliest times. Now sediment-receiving areas always 

 are but a small fraction of those exposed areas whence the sedi- 

 ments are supplied. 2 Hence in the continuous records of the 



1 See especially Geikie's Address to Sect. C, Brit. Assoc. Rep. 1899. 



2 On the strength ot the Mississippi measurements about 1 to 18 (Magee, 

 Am. Jour, of Sc. 1892, p. 188). 



