4 INTRODUCTION. 



Professor Daubree l and the late Mr. J. A. Phillips 2 have 

 shown that the wearing down and rounding of angular 

 quartz grains is an exceedingly slow operation, and that 

 grain of quartz one-fiftieth of an inch in diameter requires 

 an amount of abrasion equal to that which would result from 

 its having travelled a distance of 3,000 miles in water be- 

 fore it becomes so rounded as to assume the form of a 

 miniature pebble. In sand from the seashore of Cornwall 

 Mr. Phillips found that the quartz grains between one- 

 twentieth and one-fiftieth of an inch diameter were still 

 angular, although they had been for years exposed to the 

 beating of the waves. Only a few of the larger grains 

 were partially rounded, and the same is the case with the 

 sands of other shores. 



The sands of the African and Arabian deserts, on the 

 contrary, consist of completely rounded grains without 

 any admixture of angular fragments, and the grains of 

 blown sand from English dunes only differ in being rather 

 less completely rounded. Most sandstones consist of more 

 or less angular grains, but beds of consolidated seolian sand 

 exist in some formations. 



Pure quartzose sandstones of marine origin are always 

 shallow-water deposits and are seldom found in deeper 

 water than fifty or sixty fathoms, except where the bottom 

 slopes very steeply from the coast ; neither do they gene- 

 rally occur at a greater distance than twenty or thirty 

 miles from land, except in shallow and land- surrounded 

 seas like that which is sometimes called the German Ocean. 



Outside a continent which has a fairly uniform coast-line 

 trending for a long distance in one direction there is gene- 

 rally a regular succession of deposits from the coast-margin 

 outwards, the coarseness of the materials decreasing with 

 the distance from land, so that we pass from shingle or 



1 " Geologie ExpSrimentale," p. 256 et seq. 



2 " Quart. Journ. Geol. Soc.," vol. xxxvii. p. 21. 



