66 
FLORIDA GEOLOGICAL SURVEY—15TH ANNUAL REPORT 
of clay free from sand. In other cases velocity and current conditions 
may be such as to transport and deposit some sand with the clay result¬ 
ing in the formation of a sandy clay. 
After a clay deposit has been formed it may be modified in various 
ways. For example, the more soluble elements may be leached from it 
and carried away by circulating waters. Other mineral matter may, 
under certain conditions, be carried in and deposited by the same process. 
GEOLOGIC TYPES OF DEPOSITS 
There are in general two important types of clay deposits. One is 
termed residual because it is the residue resulting from the weathering 
or decomposition of a rock in place. The other is termed sedimentary 
or transported as the material has been transported from a former posi¬ 
tion and deposited as sediment by the action of water or wind. It is not 
always possible to distinguish these two types in the hand specimen. In 
Florida it is also sometimes difficult to apply the fundamental distinc¬ 
tions in the field as sufficient evidence is often inaccessible. 
A residual clay is found where the decay of the parent rock has gone 
on without interruption for a long period of time and where the result¬ 
ing products have not been carried away by erosion. Such deposits usu¬ 
ally take the form of a clay mantle or covering which is co-extensive 
with the parent rock. Their thickness is often very irregular depending 
upon the depth to which weathering has proceeded and the amount of 
erosion which has followed. 
Residual clays may result from a variety of sedimentary, igneous or 
metamorphic rocks. The most important residual clays are derived 
from the decomposition of rocks high in feldspathic constituents. Such 
clays, if high grade, usually must be purified by washing in order to 
remove undesirable ingredients as quartz, mica, etc. They are frequently 
highly colored by iron compounds and these are not completely re¬ 
movable. 
Deposits of residual clay are usually characterized by a gradual 
passage from' pure clay at or near the surface to the unaffected parent 
rock below. In this passage from the surface downward first a zone of 
fully formed clay is encountered which gradually passes into a zone of 
badly decayed angular rock fragments, then into a zone of only partially 
altered fragments and finally into the fresh unaltered parent rock. There 
is no sharp line of demarcation or contact between the zones above men- 
