2 BULLETIN 724, U. S. DEPARTMENT OF AGRICULTURE. 
conveniently by naming both materials, as sand-clay, sand-gumbo, 
gravelly-clay, etc. 
Clay. — Clay is a soil of very fine texture which results from the 
complete disintegration of rocks or minerals. Pure clay is very re- 
tentive of water and becomes plastic and unstable when wet. It 
is very difficult to drain, and, on that account, when it occurs in the 
subgrade of a road, the system of drainage should be designed with 
a view to preventing water from reaching the subgrade rather than 
to draining water from the clay. Most clays as they occur in nature 
contain more or less sand or gravel, which has a stabilizing effect by 
making the clay more easily drained and by furnishing a combina- 
tion of mechanically interlocldng grains. A mixture containing 
about 50 per cent each of clay and gravel or coarse sand often makes 
an excellent soil of which to construct roadbeds. Such a soil, in addi- 
tion to being easily drained, has less " capillarity " than clay, and 
consequently does not need to be drained to so great a depth in order 
to prevent water from rising to the surface. Furthermore, it is 
moderate^ stable, even when. wet. 
Sand. — Sand is made up of granular particles of mineral or stone 
which occur in nature and which will pass a J-inch mesh screen. 
Particles which are too large to pass such a screen are called gravel. 
Nearly all natural sand consists of particles which are composed prin- 
cipally of quartz, individually very hard and durable. There is no 
coherence between the different grains, however, and soils composed 
principally of sand are unstable, unless properly confined and pro- 
tected from undermining, in which case a good quality of sand may 
make an excellent subgrade for some types of road crusts and for 
practically any kind of foundation. Sandy soils usually require very 
little drainage except when the sand is mixed with a considerable per- 
centage of fine silt. Sand so fine and evenly graded that it becomes 
saturated readily with water and remains so for more or less lengthy 
periods is called quicksand and has little dependable stability. 
Loam. — A mixture of soil composed of clay or sand and a consid- 
erable percentage of decayed vegetable matter is called loam. All 
such soils usually contain both clay and sand in greater or less 
amount, and the quality of the loam for road-building purposes is 
largely dependent on the proportion in which the sand and clay are 
present. Most loams are drained comparatively easily, but are subject 
to the disadvantage that the vegetable matter they contain may be 
incompletely decomposed and subsequent decomposition eventually 
may cause more or less settlement and shrinkage. 
Gravel. — Gravel is made up of small angular or rounded particles 
of stone which occur in nature and which are sufficiently large to be 
retained on a ^-inch mesh screen. Gravelly soils are distributed 
widely in the United States and vary greatly in quality. In general. 
