228 GEOLOGICAL TIME 



Old Mortality, in repairing the tombstones, but in 

 tracing the ravages of the weather upon them, and in 

 obtaining definite measures of the rate of their decay. 



The conditions under which subaerial disintegration 

 is effected in arid climates, and the rate of its advance, 

 are still less known, seeing that most of our informa- 

 tion is derived from the chance observations of passing 

 travellers. Yet this branch of the subject is not 

 without importance in relation to the denudation not 

 only of the existing terrestrial surface but of the lands 

 of former periods, for there is evidence of more than 

 one arid epoch in geological history. Here, again, a 

 diligent examination of ancient buildings and monu- 

 ments might afford some, at least, of the required data. 

 In such a country as Egypt, for instance, it might 

 eventually be possible to determine from a large series 

 of observations what has been the average rate of 

 surface-disintegration of the various kinds of stone 

 employed in human constructions that have been freely 

 exposed to the air for several thousand years. 



Closely linked with the question of denudation is 

 that of the Deposition of the material worn away from 

 the surface of the land. The total amount of sedi- 

 ment laid down must equal the amount of material 

 abstracted, save in so far as the soluble portions of 

 that material are retained in solution in the sea. But 

 we have still much to learn as to the conditions, and 

 especially as to the rate, of sedimentation. Nor does 

 there appear to be much hope of any considerable 

 increase to our knowledge until the subject is taken 

 up in earnest as one demanding and justifying a pro- 

 longed series of well-planned and carefully executed 



