Notwithstanding the extremely minute dimensions of the chalk animalculæ, their remains form rocks of great thickness in all parts of the world. In Europe we find, for example, extensive chalk formations in England, whose Latin name Albion was bestowed on account of the white chalk cliffs occupying long stretches of the coast. The hills of Champagne consist almost entirely of chalk; and Rügen, together with many other islands, is nearly all chalk cliffs. It is only in very rare cases, however, that chalk occurs in sufficient purity to be immediately suitable for use as a pigment or writing-material. For the most part it contains other minerals, or large fossils, from which it has to be separated by mechanical treatment. Nodular flints are often met with in chalk, and many deposits contain such large numbers of the petrified shells of the sea urchin that the chalk really cannot be used as a pigment at all, by reason of the high cost of purification. The only places where chalk can be advantageously worked for the preparation of pigment is where the mineral is in a high state of purity, and also contains only very few sandy particles. Such chalk deposits are worked on a mining scale, and, as a rule, in the state in which the chalk comes from the quarry; it is in the form of a soft mass, easily scratched with the finger-nail and of fairly high density, owing to the considerable quantity of water with which it is ordinarily impregnated. In order to convert this crude chalk into a product that can be used as a pigment, it is first left to dry until the lumps can be easily broken, and then crushed into small pieces, from which all the extraneous minerals, which occur as large lumps, are sorted out and removed. This picking process is important, especially when the chalk contains flints, because these latter are very hard and would injure the millstones in the subsequent grinding. The lumps of chalk are reduced by mechanical means, such as a stamp-mill, or, more frequently, in a mill of the same type as for grinding flour, since it is impossible to get the lumps so dry as to produce the degree of brittleness necessary for a thorough reduction in a stamp-mill. The millstones are enclosed in a wooden casing, and the chalk is ground in admixture with water, the ground mass escaping, through an opening in the casing, as a thick pulp which is stored for a considerable time in large tanks. Experience has shown that this method of prolonged storage in contact with water greatly improves the colour. The only explanation of this fact is that the chalk still contains a very small amount of organic matter, which gradually decomposes in presence of water. The evidence in favour of this is the peculiar smell given off during storage.