June 19, 1919] 



NATURE 



3*5 



5 70 



% 50 • 



THE TEXTURE OF SANDS. 



ALTHOUGH chemical analyses of sands have 

 frequently been made for industrial purposes, 

 and mineral analyses are now a feature of geological 

 investigations into the petrology of sediments, com- 

 paratively little attention has been paid to the 

 mechanical composition. This is particularly the case 

 in the matter of the inter-relation of the mechanical 

 composition with either the chemical or mineral 

 constitution. 



The connection between the mineral and chemical 

 compositions of sediments is clear, the minerals repre- 

 senting the particular manner of grouping of the 

 chemical elements. The relation of the mechanical 

 composition to either the chemical or mineral con- 

 stitution opens up a big field for future work, equally 

 of considerable geological interest and immediate 

 industrial application. 



Sediments can be graded, according as they are 

 coarse or fine, by screening or by elutriation in cur- 

 rents of air or water. The latter method has almost 

 entirely replaced the older and less scientific one of 

 subsidence. If screening is to retain 

 any pretensions to accuracy, it can- 

 not be carried out with sieves of less 

 than 0-25 mm. aperture. On the 

 other hand, elutriation of material 

 more than 0-4 mm. in diameter is 

 difficult to control on account of the 

 eddies set up by the high velocities 

 of the water-currents (47 mm. or 

 more per second). In the mechanical 

 analyses of sediments the two 

 methods of procedure may be suc- 

 cessfully combined, material more 

 than 0-25 mm. in diameter being 

 sifted, and that 025 mm. in diameter 

 or less being separated into grades 

 bv elutriation in water. (Whilst 

 elutriation by air-currents has been 

 adopted commercially for grading the 

 products of fine grinding, such as 

 cement, silica-flour, barytes, etc., it 

 cannot be considered sufficiently 

 accurate for scientific work.) 



It is regrettable that no general 

 agreement exists as to the grades 

 into which sediments should be 

 separated. Soil analysts have 

 adopted a series which, for cer- 

 tain cogent reasons, ha;? not commended itself 

 to those geologists who have worked at the problem. 

 Another and different set of grades has been adopted 

 by the potters in the separation of clays. The fact 

 that comparatively few tables of mechanical analyses 

 have yet been published is the strongest possible 

 argument for agreement without delay upon a 

 definite set of grades. 



Such a division of sediments into grades is neces- 

 sarily artificial. Nevertheless, the grades adopted by 

 the geologists have as their basis the natural charac- 

 teristics of the material so-named as observed " in the 

 field." The grading system frequently adopted is : — 

 >2 mm. diameter, gravel (G) ; >i mm. and <2 mm., 

 very coarse sand (VCS) ; >o-5 mm. and <r mm., 

 coarse sand (CS) ; >o-25 mm. and <o-5 mm., 

 medium sand (MS); >oi mm. and <o-25 mm., fine 

 sand (FS); >oo5 mm. and <oi mm., coarse silt 

 or superfine sand (cs) ; >ooi mm. and <oo5 mm., 

 fine silt /fs) ; and <oot mm. diameter, clay or mud 

 (c). It is a matter of controversy as to how far 

 material of diameter less than 001 mm. may be 

 accurately separated. Both elutriation and subsidence 

 NO- ^5qO. VOL. IO3I 



methods of separadon can be arranged, but the test- 

 ing of the accuracy of the process by actual measure- 

 ment presents difficulties. A separation has, how- 

 ever, frequently been effected by subsidence methods 

 supposedly accurate at the jwint 0-005 mm. diameter. 



The grade sizes chosen are not of such great 

 moment if graphical representation of the sediments 

 by means of curves is adopted. For example, in 

 Fig. I, the method of plotting cumulative percentage 

 weights against grade size is utilised. To keep the 

 diagram within reasonable compass, the grade sizes 

 are plotted horizontally at distances proportional to 

 their logarithms. The ordinates at the grade size 

 represent for each curve the percentage weight of 

 material greater than that grade size. It is obvious 

 that if a particular sediment be subjected to mechani- 

 cal analysis upon a basis of grade sizes different from 

 the above, the results should yield the same curve. 



In the strict geological sense, the expression "sand" 

 is a grade term, and is limited to material ranging 

 from not less than 005 mm. to not more than 2 mm. 

 in diameter. A perfectly graded sand would be one 

 consisting of grains each with the same mean dia- 



Sand 



-M Slit '**- — Clay--' 



^.f ° Belgian moulding-sand 



^^^ 'rC-—?Virp^jj"' moulding-sand, St.Erth* 

 y //-corn'-' 



^. , / * 



5/ / // .tr^rit'irmould'ing-sand.Charrt^ 



1-0 0-5 0-25 0-1 0-05 0-01 mms. 



—* Grade Sizes (diameter in Millimetres) 

 Fifi. I. — Mechanical composition of sands : graphic.il representation. 



meter. Such a sand has never been found naturally, 

 but the ideal is closely approached by certain dune- 

 sands which have been exposed to the transporting 

 action of wind and water so frequently that grains 

 smaller or greater than the average have been respec- 

 tively carried ahead or left behind. 



The term " sand " in the commercial sense, how- 

 ever, is used for sedimentary material of varying 

 grades (as, for example, moulding sands), or even 

 for the unclassified products resulting from the 

 crushing of hard siliceous rocks, etc. 



The texture of sands used in the casting of metals 

 and alloys {particularly, perhaps, of steel, where the 

 conditions are the most exacting) is of considerable in- 

 dustrial importance, as well as of scientific interest. 

 When samples of the sands which have proved most 

 successful for steel moulding are subjected to 

 mechanical analysis, it is found that each consists of 

 a high percentage by weight of the sand grade, a 

 relatively large proportion of the clay grade, but little 

 or no silt or superfine sand grade. Graphically ex- 

 pressed, the mechanical composition of such sands, of 

 which " Belgian yellow " and " Cornish red " are 



