192 Rhodora [SePTEMBER 
of mixed and water-worn ‘‘till” than now cover large portions of our 
region. On these vast areas of fine and mixed soils nearly if not quite 
all the polar and boreal species could find the food-element or the 
combinations of elements upon which they most depended. For, 
even if the newly deposited soil in a given region were derived pri- 
marily from a single rock or from two rocks, a limestone and a ser- 
pentine for instance, it would require the presence in the region of 
only a very small quantity of another rock, a potassic feldspar for 
example, to support during their northward march the plants which 
require a potassic soil, as well as those which depend upon an abun- 
dance of calcium or magnesium. A yery vivid illustration of this fact 
is contained in a recent paper by Mr. Allerton S. Cushman on “The 
Use of Feldspathic Rocks as Fertilizers.” Mr. Cushman says: 
“Grinding is making surface, and it can be shown that the availa- 
bility of potash in ground feldspar increases with the surface area. 
feldspar is ground so that it will pass an 80-mesh sieve, it will of 
course contain a certain proportion of very fine particles, some of 
which approach the limits of visibility under a powerful microscope. 
If we carry on the grinding of the material from 80-mesh to 200-mesh, 
the proportion of the very small particles is enormously increased. 
The smallest particles which we need to consider here are those 
which can be measured by a micrometer device connected with the 
microscope. These smallest particles have a diameter of about 0.0001 
millimeter. Now, in order to make a specific example, we will con- 
sider the surface areas presented by 1 pound of feldspar in different 
degrees of subdivision. First, in the form of a solid cube, then broken 
down to particles that could just pass an 80- and a 200-mesh sieve; 
respectively, and, finally, in the condition it would be if it were possib! 
to grind all the material as fine as the finest particles which occur in a? 
ordinary 200-mesh powder. The 1 pound of feldspar in a solid cube 
would have a surface area of 29.3 square inches; particles capable 
of passing an 80-mesh seive would give 8,870 square inches; particles 
able to pass a 200-mesh sieve would give 24,905 square inches; @ 
if it were possible to reduce the powder to particles 0.0001 millimeter 
in diameter, there would result a surface area of 16,460,000 square 
inches. 
If it were practically possible to collect 1 ton of feldspar all in the 
*U. 8. Dept. Agric., Bur. Pl. Ind., Bull. no, 104 (1907). 
