A logarithmic grade scale devised by Johan A. Udden (1859-1932), U.S. geologist;
it uses 1 mm as the reference point and progresses by the fixed ratio of 1: 2
in the direction of decreasing size and of 2: 1 in the direction of increasing
size, such as 0.25, 0.5, 1, 2, 4. See also:
Wentworth grade scale
Source:
Dictionary of Mining, Mineral, and Related Terms
