ANNIVEESAET ADDBESS OP THE PRESIDElfT. 59 



allowance for the extra buoying-up of very minute particles by a 

 current of water, depending on surface-cohesion, the effect of 

 wearing on the form of the grains must vary directly as their dia- 

 meter or thereabouts. If so, a grain ^^j of an inch in diameter 

 would be worn ten times as much as one j-J-jj of an inch in dia- 

 meter, and at least a hundred times as much as one jo^ott ^^ ^^ 

 inch in diameter. Perhaps, then, we may conclude that a grain 

 ^ of an inch in diameter would be worn as much or more in 

 drifting a mile, as one xy^o-^- of an inch in being drifted 100 miles. 

 On the same principle a pebble 1 inch in diameter would be worn 

 relatively more by being drifted only a few hundred yards. As far 

 as I am able to judge, these conclusions agree well enough with 

 the general facts ; but yet better evidence of the actual time and 

 distance required to wear grains round is still much to be desired. 

 In the absence of positive proof, I will describe certain facts which 

 seem to throw light on this question. 



The wearing down and rounding of the original angular grains 

 is, of course, a gradual process ; and we meet with every connecting 

 link between the t^vo extremes, even in the same specimen. This 

 may be due to a mixture of grains much worn by backward and 

 forward action with those more directly drifted from their source. 

 It is therefore difficult to decide what is the true proportion of 

 angular and rounded grains in each particular case. Still this may 

 be done by counting the total number, and also the number of 

 those that are well worn, in several fields of the microscope, so as 

 to obtain a good average. This ratio, of course, represents the 

 relative total wearing, whether due to actual transport or local 

 backward and forward movement ; but in average conditions this 

 might so vary directly with the amount of transport as to make it 

 indicate approximately the relative distance of the source of supply. 



Sand luasJied from the Boulder-clay at Scarhorough. — This is 

 almost entirely fresh and angular, showing few^ or no rounded 

 grains. Hence, though the material may have travelled far, it was 

 not worn, which fully agrees with the glacial origin of Boulder- 

 clay. Some of the sand-beds of drift-age do, however, contain 

 many well-worn grains, probably owing to continued local wear 

 and tear. 



Sand from tJie modern beach at Scarhorough.— This must have 

 been subjected to the action of the waves for a considerable time ; 

 and yet it is little, if at all, more worn than that in the Boulder- 

 clay, from which probably it was in great measure derived. Until 

 I had examined the well-worn sands of the south-east of England, 

 the angularity of the Scarborough sand led me to believe that it 

 was characteristic of all such fine-grained deposits. 



Sand of the river- terraces at BunJceld. — This is just as if mainly 

 derived from schists. It is almost wholly angular, thus proving 

 that the grains are very little worn by drifting down a river. 



Sand of the Millstone Grit and Lower Goal-measures of South 

 Yorkshire. — This is as sharp and angular as if derived almost di- 

 rectly from decomposed granite, the shght corrosion being no more 



