58 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



that great errors might otherwise result. By proceeding in this 

 manner, and by examining a considerable number of specimens 

 of various geological periods and from various localities, I have 

 found that quartz sands, although to the naked eye appearing very 

 similar, may be divided into five types, which, though character- 

 istically distinct, gradually pass one into the other. They are as 

 follows : — 



1. Normal, angular, fresh-formed sand, as derived almost di- 

 rectly from granitic or schistose rocks. 



2. Well-worn sand in rounded grains, the original angles being 

 completely lost, and the surface looking like fine ground glass. 



3. Sand mechanically broken into sharp angular chips, showing a 

 glassy fracture. 



4. Sand having the grains chemically corroded, so as to produce a 

 peculiar texture of the surface, differing from that of worn grains or 

 crystals. 



5. Sand in which the grains have a perfect crystalline outline, in 

 some cases undoubtedly due to the deposition of quartz over rounded 

 or angular nuclei of ordinary non-crystalline sand. 



On the whole, then, we may say that these different types are due 

 to different kinds of mechanical or chemical changes, affecting grains 

 originally derived from crystalline rocks. 



Now I do not know what others may have thought or done, but 

 must confess that until very recently I had no idea that the differ- 

 ences between different specimens of sand were so great. I am 

 very sorry that in years gone by I did not collect a sufficient num- 

 ber of specimens to enable me to decide several interesting ques- 

 tions, and think myself fortunate in having collected what I have 

 without foreseeing their ultimate value for the purpose now in 

 hand. 



Variously ivorn Sands. 



In the first place it is most important to distinguish between 

 the age of the grains and the age of a deposit. A very ancient 

 sand bed may be made up of grains which are practically new and 

 unworn, whilst, on the contrary, the grains of a modern sea-beach 

 may be of vast antiquity, may have passed through the greatest 

 vicissitudes, may have successively formed a part of several dif- 

 ferent geological formations, and may be greatly altered and worn. 

 Unfortunately I am not acquainted with sufficient facts to prove 

 how long it would require to thoroughly M^ear down and round a 

 grain ^io- of an inch in diameter. It is evidently a very different 

 thing from the wearing down of a pebble, and may require a longer 

 period of wear than we might suspect, if we did not bear in mind 

 that, when buoyed up by water, the friction of such small particles 

 on the bottom must always be small. The following considerations 

 will serve to make this more clear. The friction on the bottom 

 must vary directly as the weight, and therefore as the cube of the 

 diameter ; but the surface from which the material must be worn 

 varies as the square of the diameter. Hence, even making no 



