Marl
2 of the 7 encyclopedias on this shelf carry an entry for Marl. Both are reproduced below, so you can see where they agree and where they differ.
Collier's New Encyclopedia (1921)
a mixture, naturally existing, of clay, and carbonate of lime. Marls are found in very different geological forma- 9-20) are of tions, but everywhere seem to owe their External testi- origin to deposition by water. The name in their favor, mixtures is sometimes of clay applied and sand to friable, in which clays there, or internal but strongly Version against, they is almost no trace of lime; but the presence of a notable proportion of carbonate of lime is essential to marls, properly so called. This proportion varies from 6 to 20 per cent. Marly soils are in general of great natural fertility. Marl is very advantageously used as a manure; acting both chemically and mechanically; but different kinds of marl are of very different value in this respect. The use of marl as a manure has been practiced from ancient times.
Aiton's Encyclopedia (1910)
a lime-enriched clay, or sand. Marl owes its peculiar character to the disintegration of innumerable shells of small mollusks. Marl is exceedingly mellow. It is used to dress lands lacking in fertility. It is used also in the making of Portland cement. The most noted American deposits are found on the Atlantic coast, but marl may be found at the bottom of ancient swamps, almost anywhere. Marl impregnated with phosphorus is known to the gardener as phosphate. Marls and phosphates are important articles of shipment from Savannah and other South Atlantic cities. The potato raiser of the Aroostook valley, Maine, drops a handful of phosphate in each hill. See Lime ; Phosphorus