In the section on dynamical geology, we learned that the carbonate of calcium or calcite is deposited from the sea-water, and limestones formed, in two ways: first, in a purely chemical way, where the water becomes saturated with calcite; and, second, organically, where the calcium carbonate is taken from the water by marine organisms to form their shells and skeletons, and the gradual accumulation of these on the ocean-floor builds up a limestone. As before stated, the difference between these two methods of deposition is not so great as it often seems, because we know that the animals never make the carbonate of calcium which they secrete, but it comes into the sea ready made with the drainage from the land. The limestones forming at the present time are almost wholly organic; but the rock known as _calcareous tufa_ is an exception. This is formed under the same general conditions as siliceous tufa, but much more abundantly, and in cold water as well as warm; because calcite is far more soluble (especially in water containing carbon dioxide) than opal or quartz. It is deposited, not only around the mouths of springs, but also along the beds of the streams which they form, enveloping stones, roots, grasses, etc., and building up usually a loose, spongy mass having a very characteristic turfaceous texture. The principal accessory minerals occurring in limestone are: (1) _kaolin_, forming argillaceous or slaty limestone, which may be recognized by the argillaceous odor and dark color; (2) _quartz_, forming siliceous or cherty limestone, known by its hardness or by the nodules of flint or chert; (3) _dolomite_, forming dolomitic or magnesian limestone, which effervesces less freely with acid; and (4) _serpentine_, forming serpentinic limestone, which is sharply distinguished by the green grains of serpentine mingled with the white calcite. A concretionary texture is common with limestone. If the concretions are small, like mustard-seed, we call the rock _oölite_; if larger, like peas, _pisolite_.