34 FLORIDA GEOLOGICAL SURVEY-EIGHTH ANNUAL REPORT. 
though modern oyster canning factories give an important added 
supply. Aside from the shell mounds of the coast there is a notable 
series of shell mounds along the St. Johns River, which consists 
chiefly of small univalve shells with some mussels. These were like¬ 
wise accumulated by the Indians. Aside from the shell accumu¬ 
lated by human agencies the shell deposits accumulated by natural 
agencies should be mentioned. Among these are coquina rock and 
other marine shell deposits. The coquina accumulated as beach de¬ 
posits and subsequently became more or less cemented, forming in 
places hard rock. Elsewhere masses of uncemented shells occur, 
such as those previously mentioned at DeLand. These are usually 
classed as shell marls. 
SAND-CLAY. 
Sand-clay, because of its widespread occurrence, is the most 
extensively used road material in the State. Almost every county 
has sandy clays suitable for road making, or the clay suitable to 
mix with or cover the sands of the natural sandy roads. These 
roads, while not all that could be desired, are a vast improvement 
over the ordinary sand roads and their cheapness recommends them 
to general use. 
Fine grained clay mixed in proper proportion with coarse, an¬ 
gular quartz, makes a road that has been found useful where cheap¬ 
ness of construction is necessary, and where the roads have light 
travel. In mixing sand and clay for road purposes the proportion 
should be so adjusted that there is enough clay in the mixture to 
fill the voids or interstices between the grains of sand. If too little 
clay is added the sand grains will lack bonding power and not make 
a solid roadbed. If too much clay is added, and the sand grains 
are widely„separated, the road behaves much as though the sand 
were not present at all. The amount of clay necessary to mix with 
a given volume of any particular sand should be determined by 
some one experienced in this work. 
Since all clay contains more or less sand, it may be expected 
that certain localities will supply clay that contains the right ad¬ 
mixture of sand and clay to form a natural sand-clay road, or so 
nearly the proper admixture that it will serve that purpose satisfac¬ 
torily. Fortunately for Florida, almost every county is supplied 
with an abundance of clay which serves admirably the purpose of 
road-making. With this material at hand, road construction in 
country sections is carried on at a minimum expense. These roads 
