128 Transactions of the Royal Microscopical Society. 
of small; whilst at the same time the rate of subsidence is less 
than that of the separate grains of sand would be, since the resist- 
ance of the water is necessarily greater for such unconsolidated 
granules. 
This collecting together of compound granules depends very 
much on the amount of material held in suspension, and also on 
whether the water be quite still or agitated; and we can, I 
think, thus easily explain why in some cases so much sand is 
mixed with the clay, whilst in others the separation is far more 
complete ; and can also explain the transport of material containing 
fine sand over very wide oceanic areas, to places far removed from 
the source of supply. 
Practical Application of the above-described General Principles. 
In studying loose deposits of sand or clay, whether in water or 
in Canada balsam, we must bear in mind that the particles will 
almost invariably rest with their flattest surfaces on the glass, 
The result of this is that their flattened character might very 
easily be overlooked. When examined in water much may be 
learned by observing the effect of change of focus, and also by 
making the grains turn round by slightly moving the covering 
glass; but when mounted in balsam the difference is best shown by 
the action of polarized hight, with an analyzer. If a grain of quartz 
or any other doubly refracting mineral be thick and of irregular 
form, the tints will rise rapidly and irregularly from the margin 
towards the centre, whereas if it be flat there will be an almost 
uniform tint over the whole surface. 
Identification of the Constituent Minerals. 
Quartz.—This may be best recognized by means of its irregular 
outline and the absence of anything like cleavage planes ; by its 
index of refraction being so nearly the same as that of Canada 
balsam, that, when mounted in this substance, there is scarcely a 
trace of shading round the exterior outline; and by the general 
intensity of its action on polarized light. ‘This of course varies 
with the inclination of the principal axis, but when in the ordinary 
average position in which the grains are seen, those of moderately 
uniform diameter in all directions give the following results : 
Grains of s'¢ of an inch in diameter or upwards seldom show 
bright colours except at the edges, the tint of the centres beimg 
pale reds and greens. 
Grains of about +}9 of an inch in diameter show the brightest 
orders of colours. 
