EARTH, SAND-CLAY, AND GRAVEL ROADS. 39 
a good quality of topsoil. These names have considerable significance 
in some localities, but may mean practically nothing in others. 
CLAY AND SAND. 
Nearly all clays, as they occur in nature, contain more or less sand. 
Those in which the percentage of sand is sufficiently high to give 
the material a character decidedly intermediate between sand and 
clay are called natural sand-clay. Ordinarily, at least 40 per cent of 
sand is required to produce this intermediate character in a pro- 
nounced degree, and from 60 to TO per cent usually is required to 
produce a first-class sand-clay surfacing material, though these per- 
centages depend to a considerable extent on the respective qualities 
of the two constituent materials. 
In general, the quality of clay is much more uncertain than that 
of sand and usually it is very difficult to determine in advance just 
what results may be expected from the use of a given clay in a 
sand-clay surface, except by means of a service test. But some time 
is required to make the service test, and, where none of the clay pro- 
posed for use occurs in an existing road surface, it may be very de- 
sirable to test the available material according to some quick method, 
so as to exclude from the service tests all clays which would not be 
likely to prove satisfactory. Some of the simplest methods for de- 
termining the probable wearing qualities of clay, or of a natural 
sand-clay mixture, by means of field inspection and laboratory tests 
are described below. 
v (1) To determine the proportion of sand present in clay, collect a 
representative sample of the clay and dry it thoroughly. After 
weighing the dry sample, place it in a vessel several times larger 
than the sample and wash out the clay content by filling the vessel 
repeatedly with water, agitating the clay and pouring off the muddy 
water. When all the clay has been washed out, dry the sand and 
weigh it. Then the proportion may be computed from the original 
weight and the weight of the sand. If the material tested happens 
to be of good quality of sand-clay, the weight of the sand will be 
about two-thirds that of the original sample. 
(2) To compare the slaking qualities of two or more samples of 
clays, make up each sample into several balls of the same weight, dry 
out the balls thoroughly, and place them in water so that they will be 
covered entirely. The balls which hold their shape longest after 
being placed in the water have the highest resistance to slaking, and 
the clay which they represent usually is to be preferred for use in the 
road surface. If this test is to be at all conclusive, however, the 
different samples should contain about the same percentage of sand, 
and if there is much doubt about the sand content, it should be 
