A PRELIMINARY report on clays oe elorida 
67 
tioned. 1 This, however, is not the case in clays residual from limestone. 
In limestone residual clays, on the other hand, there is a sudden 
change from clay to the limestone parent rock below. Most surface water, 
particularly rainwater, carries dissolved in it some carbon dioxide gas, 
(CO2) resulting in the formation of an acid, (carbonic acid, H2CO3), 
which attacks the calcium carbonate of the limestone forming a com¬ 
pound, (calcium bicarbonate, Ca(HCO'3)2), which is soluble in water. 
This process in Florida and elsewhere is greatly augmented by the addi¬ 
tion of organic acids furnished by plants and decaying vegetable mate¬ 
rial. Thus limestone is decomposed and the soluble portion carried away 
in solution. Most limestones, however, have varying amounts of im¬ 
purities, such as clay substance, which is not affected by the weak acids 
and is insoluble in water. It therefore remains as residual clay when the 
limestone is dissolved. Thus when the decomposition of the limestone 
has occurred to any depth there is a sharp contact between the clay 
and the underlying limestone because this change is not due to a gradual 
breaking down of the minerals in the rocks, as in the case of the feld- 
spathic rocks. 
Sedimentary clays are those which have been transported by the 
action of water or wind from their point of origin and deposited else¬ 
where in the form of stratified beds. Thus clay particles are swept from 
the land surface in the process of erosion and carried to lakes, estuaries, 
or the sea, etc., where they settle to the bottom of the quiet water as 
sediments. Such deposits may have no genetic relationship with the 
beds below or above. Deposits of sedimentary clay are sometimes of 
great thickness, but are more frequently rather thin. They may be of 
great purity or they may be accompanied by much impure material as 
mineral fragments (sand, mica flakes, etc.) or vegetable matter. These 
clays may often have a great lateral extent or they may be confined 
within a small area. They are at times interbedded with other sediments 
and may even be crossbedded. Some deposits exhibit marked irregular¬ 
ities of thickness, becoming thicker in one place and thinning out in 
!For a complete discussion of the processes involved in the formation of resi¬ 
dual clay see any of the following: Buckman, H. O., The Chemical and Physical 
Processes Involved in the Formation of Residual Clay, Trans. Am. Ceramic. Soc., 
Vol. 13, p. 336, 1911. 
Merrill, G. P., Rocks, Rock Weathering and Soils, p. 289, 1913. 
Ries, H., Clays, Their Occurrence, Properties and Uses, p. 7, 1908. 
