22 



j. a. ruiLLirs ON THE constitution and 



the several works, the quantities thus transported are now very 

 small. 



On the eastern "branch a certain amount of granitic sand escapes 

 into the stream almost immediately at its source ; this is repeated 

 at short intervals for a distance of a mile and a half, and finally 

 ceases a mile and a quarter above the Old Bridge, at a height of 

 about 200 feet above the weir-foot. 



The first introduction of sand into the western branch takes place 

 nearly a mile below its source, and is discontinued half a mile 

 further down its course, but at a somewhat lower level than in the 

 case of the eastern fork. 



It follows that, before arriving at the bridge, each grain of sand 

 must have travelled over a distance of at least a mile and a quarter, 

 with a fall of above 150 feet, while a portion of it has been trans- 

 ported two miles and three quarters through a channel thickly 

 strewn with granite boulders, and having a fall in that distance of 

 470 feet. 



Before arriving at the sea, therefore, the whole of the sand must 

 have travelled at least five miles and a quarter, with a minimum 

 fall of about 270 feet, while a portion of it will have been transported 

 a distance of six miles and three quarters over a total declivity of 

 584 feet. 



The first samples of material were collected from the bed of the 

 river a little below the bridge, and a mile and a half from the point 

 at which the last granitic sands are discharged into the stream. 



For the purpose of facilitating a microscopical examination of 

 these sands, they were divided into four different parcels by a series 

 of sieves, the first sieve allowing to pass through it all fragments 

 less than inch in diameter, the second those having a less diameter 

 than -Jq inch, and the third all particles having a smaller diameter 

 than inch. 



The largest fragments, retained upon the coarsest sieve were about 

 j inch in diameter, graduating to a diameter of inch. 



This sand consists of a mixture of quartz, felspar, schorl, and 

 mica, in which the last-named mineral is present in smaller pro- 

 portion than any of the other minerals. "When examined by re- 

 flected light, and magnified 20 diameters, the edges and points of 

 the different fragments of quartz and schorl are found to be sharp 

 and unrounded ; the only exception being in the case of certain 

 grains of quartz, which I)r. Sorby suggests may have been corroded 

 by the action of alkaline waters, but which may have perhaps never 

 possessed other than rounded outlines. 



The angles and edges of the felspar and mica are, on the contrary, 

 distinctly rounded ; and although this might have been anticipated 

 as regards the mica, it is at first sight not so easily understood in 

 the case of felspar, whose density and hardness differ but slightly 

 from those of quartz. It must, however, be remembered that the 

 grains of felspar had become externally kaolinized while still form- 

 ing an integral portion of the decomposed granite, and that, on the 



