Elements of geology / by Charles Lyell.

  • Charles Lyell
Date:
1838
Catalogue details

Licence: Public Domain Mark

Credit: Elements of geology / by Charles Lyell. Source: Wellcome Collection.

    58/582 page 28
    clay, of becoming plastic in water: it is a more solid form of clay, having been probably con- densed by pressure. It usually divides into thin laminae. One general character of all argillaceous rocks is to give out a peculiar odour when breathed upon, which is a test of the presence of alumine, although it does not belong to pure alumine, but, apparently, to the combination of that substance with oxide of iron.* Calcareous rocks.—This division comprehends those rocks which, like chalk, are composed of lime and carbonic acid. Shells and corals are also formed of the same elements, with the addition of animal matter. To obtain pure lime it is necessary to calcine these calcareous substances, that is to say, to expose them to heat of sufficient intensity to drive off the carbonic acid, and other volatile matter, without vitrifying or melting the lime itself. White chalk is often pure carbonate of lime ; and this*rock, although usually in a soft and earthy state, is sometimes sufficiently solid to be used for building, and even passes into a compact stone, or a stone of which the separate parts are so minute as not to be distinguishable from each other by the naked eye. Many limestones are made up entirely of minute * See W. Phillips’s Mineralogy, “ Alumine.”
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