ANNIVEBSAKT ADDEESS OE THE PRESIDENT, 55 



a true ash, and the same rock broken up after consolidation by non- 

 volcanic action. Still, on the whole, I think we may look upon the 

 vesicular structure of the constituent materials or base as charac-. 

 teristic of ash, and the more rounded and worn shape of the fra^ 

 ments as characteristic of what, for distinction, we may call volcanic 

 sand. 



Microscopical Examination of Sands and Clays. 



Having considered the origin of the material in as much detail 

 as seems desirable, I now proceed to describe the methods which I 

 have found best in studying particular examples of sands and clays. 

 On the whole, the most useful power is a J^, since that enables 

 us to see nearly all the detail ; but for examining the mere shape" 

 of grains of sand and the coarser structure of rocks, a jq or even a 

 f is more convenient. It is only occasionally that so high a power 

 as i is needed, and that chiefly to determine the true characters of 

 very minute cavities. On the whole, there can be no doubt that 

 far more maybe learned from thin microscopical sections than from 

 loose material ; but yet very much may be learned even in this 

 latter case, and in fact more respecting certain particulars. The 

 general form of the particles is best observed by viewing the sand 

 or mud in water under thin glass. Their outline is then much 

 better seen than when they are mounted in Canada balsam ; and 

 by slightly moving the cover-glass, they turn round so as to show 

 their entire shape, or slip along the surface of the supporting glass 

 in such a manner as to prove that they are thin flat plates. The 

 internal structure and optical characters of the constituent par- 

 ticles, however, are far better shown when the deposit is mounted 

 in Canada balsam. I have found it best to first spread out the 

 material on the glass in dilute gum and water. This, on drying, 

 leaves the grains sufficiently fixed on the glass to prevent their 

 moving either during the mounting in balsam or when the slide 

 is afterwards kept in a vertical position. Although when thus 

 mounted it is impossible to make the grains turn round, yet much 

 may be learned respecting their shape by observing the tints which 

 they give with polarized light. Thus, for example, a more or less 

 spherical grain of quartz sand about j^ of an inch in diameter 

 gives, on an average, tints rising from the faint white-blue of the 

 first order at the edgee up to the well-marked colours of the second 

 and third orders in the centre, whereas a grain of that diameter, if 

 it were thin and flat, would give over a large part of its surface 

 nearly uniform tints of a lower order. 



Identification of the Minerals in Sand. 



Occasionally it might require some care to distinguish between 

 flat plates of quartz and flakes of mica ; but by careful exa- 

 mination with an adequate power the more truly laminar structure 

 and parallel faces of mica enable an observer to distinguish it from 

 quartz or any other common constituent of sands or clays. There 

 is usually no serious difficulty in identifying the grains of the 



