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but the residuum, after solution, will not forma brick.

Limestone-Gravel. This is a marl mixedwith large lumps of limestone. The marl maybe either calcareous or argillaceous; but mostcommonly the former. The sandy part is alsocommonly calcareous.

Gypsum is a compound of calcareous earthand vitriolic acid : it forms a distinct species ofthe calcareous genus of fossils: of which spe-cies there are six families.

The general character of this species are,

1. Solubility in about 500 times its weightof water, in the temperature of 60°.

2. Precipitability therefrom by all mild al-kalis, and also by caustic fixed, but not bycaustic volatile alkali.

3. Ineffervescence with acids, if the gypsumbe pure; but some families of this species,being containinated with mild calx, slightly ef-fervesce.

4. Insolubility, or nearly so in the nitrousacid, in the usual temperature of the atmo-sphere.