Sir A. Geilde—Age of the Earth. 467 



waves and tides effect so much change within the space of a human 

 lifetime, the estimates of the rate of advance or retreat of the shore- 

 line are based for the most part on no accurate determinations. It is 

 satisfactory to be able to announce that the Council of this 

 Association has formed a Committee for the purpose of obtaining 

 full and accurate information regarding alterations of our coasts, 

 and that with the sanction of the Lords of the Admiralty the 

 co-operation of the Coastguard throughout the three kingdoms has 

 been secured. We may therefore hope to be eventually in 

 possession of trustworthy statistics on this interesting subject. 



The disintegration of the surface of the land by the combined 

 agency of the Subaerial forces of decay is a problem which has been 

 much studied, but in regard to whose varying rates of advance not 

 much has been definitely ascertained. The meteorological conditions 

 under which it takes place differ materially according to latitude and 

 climate, and doubtless its progress is equally variable. An obvious 

 and useful source of information in regard to atmospheric denudations 

 is to be found in the decay of the material of buildings of which the 

 time of erection is known, and in dated tombstones. Twenty years 

 ago I called attention to the rate at which marble gives way in such 

 a moist climate as ours, and cited the effects of subaerial waste as 

 these can be measured on the monuments of our graveyards and 

 cemeteries.^ I would urge upon town geologists, and those in the 

 country who have no opportunities of venturing far afield, that they 

 may do good service by careful scrutiny of ancient buildings and 

 monuments. In the churchyards they will find much to occupy 

 and interest them, not, however, like 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 information 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 

 ten-estrial surface but of the lands of former periods, for there is 

 evidence of more than one arid epoch in geological history. Hei'e, 

 again, a diligent examination of ancient buildings and monuments 

 might afifoi'd 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 awa}^ from the surface of the land. 

 The total amount of sediment 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, 

 1 Proc. Eoy. Soc. Edin., vol. x (1879-80), p. 518. 



