ANNIVEESAEY ADDEESS OP THE PEESIDENT. 57 



flint-gravel was itself mainly flint; but I found that, in some 

 cases at all events, it is chiefly of the normal kind. When polar- 

 ized hght is used, there is no difficulty in distinguishing between 

 flint and quartz grains. The latter have a simple optic structure, 

 and, when rotated, look alternately wholly dark or bright ; whereas 

 flint has a very compound crystalline structure, and at all azimuths 

 shows almost the same characteristic speckled granular appearance. 

 Jasper has a very similar structure ; and quartz containing many 

 minute crystalline granules might present a very similar appearance 

 in those few cases in which the principal axis of the crystal hap- 

 pened to lie in the line of vision. Though sometimes met with in 

 well-rounded grains, flint often occurs as thin flat chips, no doubt 

 on account of its peculiar kind of fracture. 



General Characters of Sunds. 



I have already said enough respecting the general characters of 

 the more important constituents of sands. It now remains for me 

 to consider the external form and internal structure of the sepa- 

 rate grains. In these respects different sands may differ very 

 strikingly. 



External Form of tJie Grains. 



The true character of the surface is best seen by examining with 

 transmitted light the loose material unmounted, using a condenser, 

 so as to prevent too great a shadow round the edges of the grains. 

 In observing their form we must not compare the quartz of one 

 specimen with the felspar of another, nor the larger grains of one 

 with the smaller of another. All our comparisons must be made 

 between grains of quartz of nearly the same medium size in a clean 

 condition. Tor this reason the sand should be well washed in 

 water, using a small stiff brush to thoroughly detach adherent mud. 

 A¥hen dry, the coarsest particles should be separated by one sieve, 

 and the finest by another, so as to obtain for comparison sand of 

 tolerably uniform size. The coarser sieve which I employ allows 

 grains of about -jjj of an inch in diameter to pass through ; and the 

 smaller keeps back all greater than y-^-jj, so that the average size of 

 the grains used for comparison is about j^-^j of an inch. The im- 

 portance of comparing grains of nearly the same size is at once 

 apparent when we examine the different portions thus separated. 

 The coarsest are sometimes nearly all much rounded when the 

 finest are nearly all very angular, no doubt because the friction of 

 such small particles on the bottom is so very trifling. It is the 

 grains of medium size which are the most suitable for observation in 

 comparing different specimens. 



Having thus prepared a good sample of medium quality, care 

 must be taken to mix it thoroughly, and to spread a small por- 

 tion evenly, but somewhat closely, dry, on a glass plate, kept 

 horizontal, in such a manner as to prevent the rounded grains 

 separating from the flat and angular, which they do so readily 



