Chalk
3 of the 6 encyclopedias on this shelf carry an entry for Chalk. All 3 are reproduced below, so you can see where they agree and where they differ.
Aiton's Encyclopedia (1910)
a soft white rock or earth. Chemically considered chalk is a carbonate of lime. It is formed under water by the accumulation and disintegration of tiny shells of myriads of microscopic animals. If waters are undisturbed for ages these deposits of chalk obtain great thickness, when changes in the earth's surface may bring them above the ocean to form part of a continent. The geologic period during which the great chalk beds were laid down is called the Cretaceous Age. Deep sea dredgings show that a fine white ooze is being formed now at the bottom of the ocean in a way probably very similar to that in which chalk was formed. Chalk formations usually contain fossils of seaweeds, sponges, corals, mollusks, fishes, and reptiles. One notable chalk deposit may be followed from Austin, Texas, southwestward into Mexico. One ledge, no less than 600 feet in thickness, may be traced for 250 miles. The best known chalk formation reaches from northern France into the southern parts of England and Ireland. Chalk cliffs border both shores of the English Channel . The high, white cliffs of Ireland, with their green carpet above, are the first land that greets the eye of the American bound for Liverpool. Chalk lacks the fertility of lime, but is used in England to dress fields deficient in the latter. It holds water well, making an excellent subsoil. It is much used in making cement, in sizing some kinds of cloth, and small quantities are molded into cylinders or crayons for blackboard use. The whiting used by the calciminer is a refined chalk. The so-called red chalk, the "keel" found by boys in clay banks, is a kind of clay stained with iron. In medicine purified chalk is administered as a remedy for an acid stomach. Such expressions as "chalk up," "by a long chalk," etc., arise from the use of chalk in keeping accounts. See Lime ; Gypsum
Collier's New Encyclopedia (1921)
a well-known earthy limestone, of an opaque white color, soft, and admitting no polish. It is an impure carbonate of lime, and is used as an absorbent and anti-acid, and for making marks for various purposes, as on the blackboard in schools, and by artisans and others. Black chalk is a soft variety of argillaceous slate. Brown chalk, a familiar name for umber. Red chalk, another name for ruddle. French chalk, steatite or soap-stone, a soft magnesian mineral. Drawing Drawing chalks were origioriginally restricted in colors to white, black, and red, but now chalks of every color are used, and are known by the name of crayons, commonly made of calcium sulphate and not the carbonate, as supposed. In geology chalk is the rock which forms the higher part of a series or group of strata, comprising rocks of different kinds, termed the cretaceous system. Scotch
American Watchmaker and Jeweler (1892)
To prepare chalk for use for cleaning gilding, etc., pulverize it thoroughly and then mix it with clean water, in proportion of two pounds to the gallon. Stir well and then let it stand about two minutes. In this time the gritty matter will have settled to the bottom. Slowly pour the water into another vessel, so as not to stir up the sediment. Let stand until entirely settled, and then pour off as before. The settlings will be prepared chalk, ready for use as soon as dried. Spanish whiting, treated in the same way, makes a very good cleaning or polishing powder. Some watchmakers add a little crocus; it gives the powder a nice color at least.
What Has Changed in the Last Century
The chemistry in these entries still holds: chalk is a soft, fine-grained carbonate of lime laid down in warm shallow seas. What has shifted is the biology and the clock. Aiton's Encyclopedia of 1910 credited "tiny shells of myriads of microscopic animals"; chalk is now known to be built overwhelmingly from coccoliths, the calcite plates of single-celled algae called coccolithophores, with foraminifera a lesser contributor. Radiometric dating has also pinned the Cretaceous to roughly 145 to 66 million years ago, ending with the Chicxulub asteroid impact, a link only proposed in 1980. Two uses have grown in importance since 1921: the Chalk aquifer of southeast England is now a major source of public drinking water, and school "chalk" is mostly gypsum, as Collier's had already noticed, and is fast being displaced by whiteboards and screens.
Written for this edition, 2026.