A PRELIMINARY REPORT ON CLAYS OF FLORIDA 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, (C02) resulting in the formation of an acid, (carbonic acid, H2CO3), which attacks the calcium carbonate of the limestone forming a compound, (calcium bicarbonate, Ca(HCO3)2), which is soluble in water. This process in Florida and elsewhere is greatly augmented by the addition of organic acids furnished by plants and decaying vegetable material. Thus limestone is decomposed and the soluble portion carried away in solution. Most limestones, however, have varying amounts of impurities, 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 feldspathic rocks.
 Sedimentary clays are those which have been transported by the action of water or wind from their point of origin and deposited elsewhere 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 irregularities of thickness, becoming thicker in one place and thinning out in

 'For a complete discussion of the processes involved in the formation of residual clay see any of the following: Buckman, H. 0., 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.