254 PRINCIPLES OF STRATIGRAPHY 



of current, grains below a certain size will not be rounded. Since 

 the buoyancy or surface tension (adhesion between water and sand) 

 of salt water is greater than that of fresh water, the smallest 

 rounded grain of the former should be somewhat larger than that 

 of the latter, other things being equal. Again, since the surface 

 tension of air is very much less than that of water, very much 

 smaller grains will be rounded by wind than by water. In other 

 words, wind-blown sands of even extremely small size will come in 

 contact with each other and with stationary objects, and so become 

 worn and rounded. The fact that all grains below a certain size 

 show angularity, and that the transition from rounded to angular 

 grains is not a gentle but an abrupt one, is readily noticeable in 

 both modern sands and ancient sandstones. Moreover, grains of 

 materials of different specific gravity, but of the same size, will 

 show a greater rounding with higher specific gravity. (Mackie- 

 4y:2p8.) Ziegler (75) noted from experiments that quartz par- 

 ticles less than i mm. in diameter showed repulsion due to the 

 viscosity of the liquid. He concludes that it is impossible that 

 grains less than 0.75 mm. in diameter could be well rounded under 

 water, but if rounded must be wind-worn. 



Since mineral particles have less weight in water than in air, it 

 follows that particles of the same size and material will suffer more 

 erosion in air on this account also. 



Hardness and the distance over which the material is trans- 

 ported are likewise factors in the rounding of grains, the amount 

 of rounding varying indirectly as the former and directly as the 

 latter. Mackie has reduced the variability of the rounding (R) of 

 particles to the following formula : 



size X the specific gravity X distance 



Roc 



hardness 



Distance (d) traveled may be expressed by the number of times 

 the body turns on its axis. This number of times for a cube with 



side measuring x will be and since the weight of such a cube is 



4x 



expressed by x^ X sp. gr. {i. e. the size or volume x multiplied by its 



sp. gr.) we have 



- 



_ ^ ^^^' 4x x^spg.d „ x^spg.d 



Roc — = — ^-^— or generally — - — 



h 4h mh 



where m varies with the outline of the figure, being 4 in a cube, and 

 3.1416 in a sphere, with proportional values for other forms. Since 



