ADDEESS. 2] 



surface of the land are not everywhere equally energetic. They are 

 naturally more vigorous where the rainfall is heavy, where the daily 

 range of temperature is large, and where frosts are severe. Hence they 

 are obviously much more effective in mountainous regions than on plains ; 

 and their results must constantly vary, not only in different basins of drain- 

 age, but even, and sometimes widely, within the same basin. Actual 

 measurement of the proportion of sediment in river water shows that 

 while in some cases the lowering of the surface of the land may be as 

 much as y^ of a foot in a year, in others it falls as low as ij-g'oo^. In 

 other words, the rate of deposition of new sedimentary formations, over 

 an area of sea-floor equivalent to that which has yielded the sediment, 

 may vary from one foot in 730 years to one foot in 6,800 years. 



If now we take these results and apply them as measures of the length 

 of time required for the deposition of the various sedimentary masses 

 that form the outer part of the earth's crust, we obtain some indication of 

 the duration of geological history. On a reasonable computation these 

 stratified masses, where most fully developed, attain a united thickness of 

 not less than 100,000 feet. If they were all laid down at the most rapid 

 recorded rate of denudation, they would require a period of seventy- 

 three millions of years for their completion. If they were laid down 

 at the slowest rate they would demand a period of not less than 680 

 millions. 



But it may be argued that all kinds of terrestrial energy are growing 

 feeble, that the most active denudation now in progress is much less 

 vigorous than that of bygone ages, and hence that the stratified part of 

 thie earth's crust may have been put together in a much briefer space of 

 time than modern events might lead us to suppose. Such arguments are 

 easily adduced and look sufficiently specious, but no confirmation of them 

 can be gathered from the rocks. On the contrary, no one can thought- 

 fully study the various systems of stratified formations without being 

 impressed by the fulness of their evidence that, on the wtole, the accu- 

 mulation of sediment has been extremely slow. Again and again we 

 encounter groups of strata composed of thin paper-like laminae of the 

 finest silt, which evidently settled down quietly and at intervals on the 

 sea bottom. We find successive layers covered with ripple-marks and 

 sun-cracks, and we recognise in them memorials of ancient shores where 

 sand and mud tranquilly gathered as they do in sheltered estuaries at the 

 present day. We can see no proof whatever, nor even any evidence which 

 suggests, that on the whole the rate of waste and sedimentation was more 

 rapid during Mesozoic and Pateozoic time than it is to-day. Had 



