24 



J. A. PHILLIPS ON THE CONSTITUTION AND 



is situated at a distance of half a mile west from the present mouth 

 of the stream by which the sands were brought down. They were 

 collected from the water's edge at half-tide ; and as the discharge of 

 granitic sands into the river has been very small during the last ten 

 or twelve years, and this point is considerably removed from its 

 mouth, it is evident that a large proportion of the grains taken must 

 have been subjected during many years to the wearing action of the 

 waves. 



An examination of this sand shows that quartz having a diameter 

 between and -±y inch is usually angular, although some of the 

 larger pieces are distinctly (but not considerably) rounded. The 

 schorl, like the quartz, generally presents sharp angles, although 

 somewhat abraded grains of this mineral are occasionally met with. 

 Nearly all the felspar is rounded to a considerable extent, as is also 

 the small quantity of mica which is present. 



Below -e^j inch in diameter the angularity of the fragments of 

 quartz and schorl is perfect, with the exception of occasional 

 " corroded " grains ; the felspar has, for the most part, rounded 

 outlines ; and mica is almost entirely absent. 



At page 32 of his Address to the Geological Society of London 

 (1880) Dr. Sorby remarks : — " Unfortunately I am not acquainted 

 with sufficient facts to prove how long it would require to thoroughly 

 wear down and round a grain inch in diameter. It is evident it is 

 a very different thing from the wearing 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 be always small." Again, at page 34, 

 he says : — " It appears to me sufficiently proved that a great amount 

 of drifting and mechanical action must be required to wear down 

 angular fragments of quartz into rounded grains y-J-^ inch in diameter, 

 which I have taken as the standard for comparison." 



Professor Daubree states that the diameter of grains capable of 

 floating in slightly agitated water is about millimetre, or, say, 

 ^-^j inch, and remarks that all smaller grains must of necessity 

 remain angular*. He subsequently says that a current or wave 

 capable of carrying off in suspension particles of that diameter, with- 

 out in any way affecting their form, would cause larger fragments 

 of the same mineral to so rub one against another as gradually to 

 produce rounded sand. According to an experiment quoted by this 

 author, a sand of which the grains have a diameter of T 5 ^ millimetre, 

 say inch, to which a movement of one metre per second is im- 

 parted, becomes rounded, with a loss equal to yxnhnr of its weight 

 per kilometre traversed. 



This experiment appears to indicate that a grain of quartz inch 

 in diameter requires, before becoming completely rounded and 

 assuming the form of a miniature pebble, an amount of abrasion 

 equal to that which would result from having travelled a distance 

 of three thousand miles. In arriving at this conclusion the fact 

 must not be lost sight of that, after the first rounding of the 

 * G-eologie Experimental©, p. 256. 



