288 PRINCIPLES OF STRATIGRAPHY 



pebbles. Laufer and Wahnschaffe call grade 4 of Keilhack's scale 

 fine sand and both 5 and 6 very fine sand. Wollny makes his 

 medium sand from 0.5 to 0.25 and his fine sand 0.25 to o.i. From 

 0.1 to 0.05 he calls coarse silt, from 0.05 to 0.025 medium silt, from 

 the last to 0.005 "''"''• ^"e silt, and below that to 0.00 1 mm. colloidal 

 clay. Orth, Laufer, etc., call 0.05 to o.oi mm. dust, and everything 

 below that finest dust. Wollny calls fragments from 5 to 2 mm. 

 medium gravel, from 10 to 5 coarse gravel and above 10 mm. stones. 



Thoulet has given these dimensions, gravel: coarse, 9.0 mm. ; 

 medium 4.5 mm.; fine 3.0 mm.; sand: coarse (a) 1.32 mm., (b) 

 0.89; medium (a) 0.67, (b) 0.54, (c) 0.45; fine (a) 0.39, (b) 0.34, 

 (c) 0.30, (d) 0.26 mm.; very fine 0.04 mm. Silt below 0.04 mm.* 



It will be seen that Crosby and Thoulet differ most from the 

 others, and also from each other. Their definite sizes are less sat- 

 isfactory than the ranges given by the others. f 



Types of Sands Based on Origin. 



Sherzer (26) has recently proposed to subdivide sands according 

 to their mode of origin into 7 groups, each with a number of sub- 

 groups. They are herewith given, separated into clastic and non- 

 clastic sands, and each is referred to its proper place in the classifi- 

 cation adopted in this book. Three additional types are added to 

 make the series complete (Grabau-io: 1006). 



A. Clastic sands {Exogenetic). 



1. Glacial sand type J Autoclastic = autoarenyte 



2. Volcanic sand type Pyroclastic = pyroarenyte 



3. Residual sand type Atmoclastic =atmoarenyte 



4. Aqueous sand type Hydroclastic = hydroarenyte 



5. ^olian sand type Anemoclastic = anemoarenyte 



6. Artificial sands (added) Bioclastic = bioareny te 



B. Non-clastic sands {endogenetic, see page 283). 



7. Organic sand type Biogenic sand (biogranulyte) 



8. Concentration sand type Hydrogenic sand (hydrogranulyte) 



9. Snow and firn sand (added) Atmogenic sand (atmogranulyte) 



10. Lapilli or igneous sand (added).. . , Pyrogenic sand (pyrogranulyte) 



* These grades are numbered i, 2, etc., by Thoulet. 



t For further analyses see the table given under wind transportation, p. 59. 



X In this he includes the material resulting in the manufacture of talus in 

 avalanches, rock slides, rock and mud flows, and earth movements along joint 

 planes. All except the first, which is atmoclastic, belong vmder the autoclastic 

 group with the glacial sand type. 



