HISTORY OP GRITS AND SANDSTONES. 



23 



removal of this coating of china-clay by washing, a rounded central 

 nucleus will remain. 



Sand which had passed through the J^-inch apertures of the first 

 sieve, hut which was retained by the ^_-inch openings of the second, 

 has a similar composition to that of coarser grain. The quartz and 

 schorl are both angular ; the felspar is more rounded than the quartz ; 

 and the mica, which is more plentiful than in the coarser sand, is 

 much worn on the edges. 



The sand, which after passing through the ^-inch sieve was 

 retained on the j-L-inch sieve, consists of a mixture of angular 

 quartz, unworn crystals of tourmaline, grains of felspar (some of 

 which are rounded) and flakes of mica (which are much worn on 

 the edges). 



An examination of the material which passed the sieve having 

 apertures y-jLy inch in diameter, shows that its grains are entirely 

 unwaterworn. The proportion of mica is much larger than in the 

 coarser sands, and there is less felspar ; hut most of the grains have 

 had their angles removed. The quartz is generally in the form of 

 tabular flake-like fragments, while the schorl often occurs as small 

 acicular crystals. 



The quartz of these sands is frequently penetrated by needles of 

 schorl ; and when mounted in balsam it is seen to be full of fluid- 

 cavities containing bubbles — in this respect differing entirely from 

 the quartz of the Cornish grits, as well as from that of the majority 

 of sandstones. 



Specimens of the sandy deposit were taken from down the course 

 of the river, at intervals of a mile apart, the last having been obtained 

 at a point slightly above the sea-level at high water at Pentewan. 

 In every case, however, they so exactly resembled those first taken 

 from below the Old Bridge at St. Austell as not to require detailed 

 description. The quartz and schorl are angular, the felspar is more 

 or less rounded, and the larger flakes of mica are worn at the edges. 

 With regard to the distribution of sand along the river-bed, it is 

 needless to remark that the coarser fragments are found towards the 

 centre of the stream, while the finer silt, with minute flakes of mica 

 and quartz, accumulates in less rapidly moving currents near the 

 banks. 



Having found that quartz grains below ^ inch diameter are not in 

 the slightest degree rounded by a minimum transit of five miles 

 and a quarter down the course of the stream, it was thought desirable 

 to ascertain the effect of a prolonged action of the waves upon the 

 sand lying on the sea-shore. 



It must be here remarked that since the first opening of china- 

 clay works in this district, now about sixty years since, millions of 

 tons of granitic sand have been carried into the sea by the streams 

 into which it was discharged. The effect of this at Pentewan has 

 been to silt up the harbour to a very serious extent ; while the whole 

 of the sands upon the sea-beach bear evidence of having been 

 derived from the same source. 



The point from which the specimens were taken for examination 



