252 FLORIDA GEOLOGICAL SURVEY-I5TH ANNUAL REPORT

and, therefore, any expenditure of time or money in investigating the deposits is useless. These tests are of no value if the clay is being considered for Portland cement manufacture.
 These tests are simple and easily applied, but the results must be interpreted with caution. For example, a relatively non-plastic clay may be satisfactorily worked in a dry-press machine, or it may be possible to add quantities of other plastic clays, thus making a blend which has sufficient plasticity to be worked by the stiff-mud process.
 The plasticity of the clay may be judged by taking a small moist lump and working it in the hands. If it can be easily molded into various shapes which are retained without cracking as the piece is allowed to dry, the plasticity may be considered satisfactory. The presence of sand, or other gritty matter, may be easily detected by tasting a small piece of the clay. A small amount of sand or other non-plastic material is not necessarily deterimental for use in structural materials, while, in high-grade clays, such impurities as sand, etc., may be profitably removed by washing. Calcium carbonate (lime) may be-detected by applying a few drops of hydrochloric acid to the clay, which will cause it to effervesce if lime is present. Calcium carbonate is detrimental, causing the product to slake after firing.
 By far the safest and most satisfactory procedure is to send a clean sample of the clay, weighing about twenty or twenty-five pounds, to some ceramic laboratory making'a specialty of testing clays, where the physical properties of the clay can be accurately determined and its adaptability to various uses and types of machinery can be ascertained. Many concerns manufacturing clay-working machinery make a practice of testing small samples of clay submitted by prospective purchasers of equipment, and render an opinion as to the suitability of the clay for use in the types of machinery manufactured by them. Also, arrangements may oftentimes be made with a brick manufacturer at some distance, so that he will not fear competition, whereby the clay-owner may ship a sample of clay, usually several barrels, to be tested by the manufacturer for working and burning in his plant. This method has the disadvantage that the clay to be tested may require entirely different treatment from that given the clay used at the plant where the test is being made, and, in many cases, the conclusions derived would be erroneous. High-grade clays can only be recognized by the method first listed.
 After the presence of a clay deposit has been ascertained, it is next of prime importance to know its thickness and the extent of the area