EXOGENETIC ROCKS 



287 



Eventnall)' this classification was published by Merrill in 1898 

 (20:^80), the only difiference being that he gives the range of fine 

 gravel as between 2 and i millimeter, and classes everything above 

 2 mm. as gravel. Crosby {^:20^^) has given a somewhat different 

 valuation for some of the types as follows : 



Fine sand : 0.45 mm. ; superfine sand : 0.28 mm. ; quartz flour : 

 0.16 mm. ; superfine quartz flour : 0.08 mm. 



Keilhack (16:5^7, 5-^) gives the following classification, accord- 

 ing to size of grain: /, grains above 2 mm. diameter: gravel; 2, 

 grains from 2 to i mm. diameter: very coarse sand; j, "grains from i 

 to 0.5 mm. diameter: coarse sand; 4, grains from 0.5 to 0.2 mm. 

 diameter: medium sand; 5. grains from o 2 to o.i mm. diameter: 

 fine sand; 6, grains from o.i to 0.05 mm. diameter: superfine sand; 

 1, grains from 0.05 to o.oi diameter: dust; 8. grains smaller than 

 o.oi mm. diameter: finest dust. Nos. ^ to d inclusive were classed 

 merely as sands ; the varietal names are here added. 



It will be seen that the subdivisions of the sands here given cor- 

 respond very closely to those selected by the New York engineers, 

 who, however, place the grains above i mm. in diameter into the 

 category of gravel. No. 7, rock flour of the engineer's table, also 

 corresponds to No. 7, dust of Keilhack's table. 



From these analyses, we may construct the following table, 

 which may serve as a standard for comparison : 



Table of standard si::es of rock fragments. 



1. Boulders above 150.0000 mm. 



2. Cobbles 150.000 to 50.0000 mm. 



3. Very coarse gravel ... . 50.000 to 25.0000 mm. 



4. Coarse gravel 25.000 to 5.0000 mm. 



5. Fine gravel 5.000 to 2.5000 mm. 



Texture rudaceous; on 

 consolidation forming 

 rudytes. 



6. Very coarse sand (or 



very fine gravel) ... 2 . 500 to i . 0000 mm. 



7. Coarse sand i .000 to 0.5000 mm. 



8. Medium sand 0.500 to 0.2500 mm. 



9. Fine sand 0.250 to o. 1000 mm. 



10. Superfine sand o. 100 to 0.0500 mm. 



Texture arenaceous; on 

 consolidation forming 

 arenytes. 



11. Rock flour 0.050 to o.oioo mm. 



12. Superfine flour o.oio to 0.0050 mm. 



13. Clay size 0.005 to o. 0001 mm. 



Texture lutaceous; on 

 consolidation forming 

 lutytes. 



Orth (22), Laufer (18) and Wahnschafife (29) are in essential 

 agreement with Keilhack, and so is E. Wollny. Orth calls material 

 from I mm. to 3 mm. very coarse sand and fragments, above 3 mm. 



