38 BULLETIN 463, U. S. DEPARTMENT OF AGRICULTURE. 
cases, very thin. The usual variation in the depth of this layer is 
from 4 to 12 inches. A suitable quality of topsoil is produced some- 
times by agencies other than cultivation, when the depth may be 
considerably greater than 12 inches. 
The safest guide in identifying topsoil that will make a satisfactory 
road surface is the service test, and, fortunately, it is nearly always 
possible to make this test by simply observing the condition of a 
section of road already constructed of the soil in question. The best 
method to follow in making a selection is to pick out such sections 
of earth road in the vicinity of the one to be surfaced as remain con- 
tinuously in good condition. Then, if practicable, obtain the soil in- 
tended for the new road from the fields adjacent to the best sections 
of this good earth road. Otherwise employ specimens of the soil from 
these fields to compare with other available soils and base the selec- 
tion on such comparisons. When comparisons are not conclusive the 
various lands of available soil should be tried out on a small scale 
before any considerable area of topsoil surface is constructed, because 
there is at present no other reliable method for comparing the relative 
excellence of different topsoils and it is very easy to be misled into a 
wrong selection by incomplete laboratory tests. 
The following list of characteristics usually belonging to topsoil 
of a satisfactory quality at least may prove helpful in selecting soils 
upon which to make the service test referred to above : 
1. Some of the best topsoils so far discovered are of granitic ori- 
gin : that is, the soil has been formed by the decomposition of granite. 
2. Good topsoil usually has a very " gritty " texture, and when 
rubbed between the fingers this characteristic should seem more pro- 
nounced than any other. 
3. Samples of soil taken from a number of first-class topsoil roads 
and subjected to mechanical analysis have shown an average sand 
content of from 50 to 75 per cent and an average clay content of from 
25 to 50 per cent. But it appears that satisfactory results have been 
obtained most frequently where the sand content ranged between 65 
and 70 per cent and the clay content between 30 and 35 per cent. 
4. The color of good topsoil is more frequently gray than other- 
wise, but color is by no" means an index to quality. Several cases 
have been reported where excellent results were obtained with a soil 
of dull-red color, while, again, a dark-brown soil has proved entirely 
satisfactory. Usually the color of the soil gradually becomes lighter 
after it is placed on the road, and some of the best gray soils have 
become almost white after a few years' service. 
5. The soils designated ordinarily by farmers as M gray grit," 
" bright tobacco soil." "upland soil." and " rotten granite " have 
been found very frequently to possess the proper characteristics for 
