66 FLORIDA GEOLOGICAL SURVEY-I5TH 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 resulting 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 position 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 distinctions 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 resulting products have not been carried away by erosion. Such deposits usually 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 removable.
 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-