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pieces; it also more slowly moulders by ex-posure to the air and moisture, if of a looseconsistence: it hardens when heated, and formsan imperfect brick. It effervesces with spiritof nitre or common salt, but frequently refusesto do so with vinegar. When dried and pro-jected into spirit of nitre, in a Florence flask,with the attentions above-mentioned, it is foundto lose from S to 10 per cwt. of its rveight.The undissolved part, well washed, will, whenduly heated, harden into a brick.

Silicious, or Sandy Marls, are those whoseclayey part contains an excess of sand: for, iftreated with acids in the manner above-men-tioned, the residuum, or clayey part, will befound to contain above 75 per cwt. of sand;sequently chalk and sand are the predominantingredients.

The colour of this marl is brovrnish grey,or lead coloured : generally friable and fiakey,but sometimes forms very hard lumps. It doesnot readily fall to pieces in water. It chipsand moulders by exposure to the air and mois-ture, but slowly. It effervesces with acids;