1) When calcium carbonate was formed in the laboratory from the calcium bicarbonate solution through which carbon dioxide gas had not been bubbled during the process of the calcium carbonate formation: a) Only calcite was formed at a temperature below 25°C; aragonite was formed above 25°C, and vaterite was formed above 40°C. Between 40 and 60°C a large amount of vaterite was formed. With a rise in the temperature of the mother solution, the proportion of calcite decreased, b) Vaterite was easily formed from the calcium bicarbonate solution containing sodium chloride at a high temperature. With an increase in the temperature and in sodium chloride concentration, the proportion of vaterite increased, c) With an increase in both temperature and magnesium chloride concentration, the proportion of aragonite increased but that of calcite decreased. The presence of magnesium chloride in the mother solution hindered the formation of vaterite.<BR>2) When calcium carbonate was formed in the laboratory from the calcium bicarbonate solution containing sodium chloride (or magnesium chloride) and bubbled with carbon dioxide gas continuously during the calcium carbonate formation, only calcite was formed despite a high temperature.<BR>3) When calcium carbonate was deposited in thermal spring waters of a high temperature, aragonite was formed. However, from the waters into which carbon dioxide gas was supplied, in other words, from the acidic solution, calcite was formed despite a high temperature, and aragonite was formed from the waters to which carbon dioxide was not supplied, in other words, from the basic waters, despite a low temperature.<BR>4) Up to the present time, vaterite has not been found in the deposits of thermal springs because: a) Calcium carbonate deposits of thermal springs were either formed at a location where carbon dioxide gas was supplied to the mother waters or where the concentration of magnesium ions in the waters was rather high, or b) even if vaterite had been formed, it would have changed to calcite or aragonite very quickly because of its low stability in solution.<BR>5) In spring waters with the same chemical constituents, pH value, and temperature, the proportion of aragonite increased and that of calcite decreased with an increase in the rate of carbon dioxide gas escape from the waters and in the rate of calcium carbonate formation.<BR>6) Aragonite was readily found in calcium carbonate precipitates formed floating on the surface of thermal spring waters, but it was not likely to be found in the precipitates deposited on the wall or bottom in the same waters.<BR>The polymorphic formation of calcium carbonate formed in thermal springs depends mainly on the temperature, the concentrations of sodium ions, magnesium ions and carbon dioxide (pH) in the waters, and on the rate of calcium carbonate formation when calcium carbonate is formed from a similar water.