254 SCIENCE PROGRESS 



worth. In any case, we require more experiments, and more 



carefully chosen experiments, before we can lay any stress on 

 the results that have been obtained. 



... 



The principal point it is necessary to emphasise is that the 

 rate of erosion, when we have got it, is a very useful guide to 

 the rapidity of geologic process. Unfortunately it is the case 

 that the enormous variations that are known to exist are not 

 yet correlated with the configuration of the country or with 

 any other known cause. Thus we cannot, with any confidence, 

 apply our averages to particular cases. But, taking our present 

 information for what it is worth, it is surprising that geologists 

 do not apply it directly, instead of indirectly. The formation 

 of sedimentary rock is a variable and uncertain process. It is 

 liable, not only to extreme variations, but to actual reversal, 

 without always leaving obvious indications. The rate of 

 erosion is, comparatively, a constant quantity. Let the geolo- 

 gists, therefore, endeavour to ascertain the amount of erosion 

 which has occurred at particular places and in particular 

 geologic epochs. Instead of measuring deposition, let us 

 measure erosion. We shall not then be encumbered by in- 

 soluble conundrums concerning the ratios of the areas of 

 denudation and deposition. 



Some facts are now available which bear directly on this 

 particular problem. One very interesting research dates back 

 to 1845. In the course of a thorough survey of a district in 

 South Wales, the late Sir Andrew Ramsay discovered evidence 

 of extensive denudation. His arguments are somewhat difficult 

 to follow, and the conclusions concerning erosion are not clearly 

 classified and tabulated, but a chance example will show how 

 extreme erosion has been. It is stated that unconformable beds 

 of New Red marl overlie strata which show a denudation of at 

 least 5,000 feet between that time and the laying down of the 

 Carboniferous limestone. It is stated as probable that some 

 thousands of feet of coal measures may also have been eroded. 

 This has taken place in only a part of two adjacent geologic 

 epochs. This is, unfortunately, local, as distinguished from 

 general or average erosion, but if we allow more than double 

 the very highest estimate of general erosion, and assume that it 

 took place at the rate of a foot in a thousand years, we have a 

 minimum of 5,000,000 years for less than a single recognised 

 geologic epoch. 



