634 W. H. SHERZER RECOGNITION OF TYPES OF SAND GRAINS 



traveled to be uniform, the author constructs a table showing the relative 

 roundability ("psephicity^') of the common rock-forming minerals for 

 both water and air. Although the figures must be regarded as only 

 approximations, they are of interest in connection with the present dis- 

 cussion, and a portion of the table is here reproduced. 



Mackie's Table showing the relative Capacity of Minerals to become rounded. 



Mineral. In water. In air. 



1. Quartz .23 .38 



2. Orthoclase .29 .40 



3. Labradorite .29 .45 



4. Hornblende .39 .57 



5. Biotite .70 1.05 



6. Muscovite .86 1.80 



7. Magnetite .70 .86 



8. Garnet .39 .53 



9. Tourmaline .30 .43 



10. Zircon .45 .59 



11. Rutile .51 .68 



These figures are to be interpreted as meaning that the capacity for 

 being rounded in water is about 1 2/3 times greater for hornblende and 

 garnet grains than for quartz, providing the grains are of approximately 

 the same size and shape and have been subjected to the same amount of 

 water action. 



In comparison with glacial and residual sands and many samples of 

 the volcanic type, the aqueous sands show marked evidence of erosion 

 about the corners and edges. When the action has been relatively slight 

 in amount the quartz grains, however, as well as garnet, magnetite, and 

 some other harder minerals, may show sharp points and edges and fresh 

 conchoidal fracture (see figures 1 and 2, plate 44). Much variety of 

 material is to be expected, especially if the granules are derived from 

 glacial or residual deposits. Subjected to further action, the softer, 

 cleavable, and slowly soluble or decomposable minerals begin to be re- 

 duced in size and gradually eliminated, while the harder show evidence 

 of wear (see figures 3 and 4, plate 44). In a paper presented to the 

 Edinburgh Geological Society, in 1896, Mackie describes the gradual 

 elimination of feldspar from the river sands, samples for study being 

 collected from various points along the courses of the streams.^^ In the 

 case of the four principal rivers studied the average amount of feldspar 



1^ Mackie : Tlie sands and sandstones of eastern Moray. Transactions of the Edin- 

 burgh Geological Society, vol. vii, p. 148. See also "Studies for students," by Barrell. 

 Journal of Geology, vol. xvi, 1908, p. 364. 



