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*9 2 Arglllaceous Genus. Effervefce with the mineral acids, but fre- quently refufe to do lo with vinegar. Moulder and crumble by expofition to the air more flowly than calcareous marls, fometimes requiring 5 or 6 years. When heated they harden into an imperfedt brick, which is flowly difintegrated by water. In a greater heat they form a flagg. However, if they contain a large proportion of fand, the bricks formed by them are quickly difintegrated by water. They contain mild calcareous earth, in the proportion of, fi om to -^-of the whole. To Withering, of Birmingham, we are indebted for many well-direbted experiments on thefe marls. Ile found, ift, That a red compafl marl, that was fo plaflic as to form and burn to a brick, contained of - its weight of mild calx. 2d, A grev, in bard lumps, was alfo plaflic, but gritty, and formed a foft brick; it contained 4 of its weight of mild calx * 3d, A red, with grey fpots, and very hard, contained about -J- of aeratcdcalx; yet it was plaflic and burned to a grey hard ftone. Mr. Andreas publifhed a good diflertation on marls in German, in the year 1769, in which he diftinguifhed marls as follows: ift, Marl, properly fo called, confifting of cqual parts of clay and mild calx, that is, chalk. 2d, Calcareous marl, confifting of three parts chalk and one of clay. * Phil. Tranf. vol. LXIII. 3 d >