A geological manual / By Henry T. De La Beche.

  • Henry De la Beche
Date:
1833
Catalogue details

Licence: Public Domain Mark

Credit: A geological manual / By Henry T. De La Beche. Source: Wellcome Collection.

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    the solid superficial density. Daubuisson infers from the ob- servations of Maskelyne, Playfair, and Cavendish, that “ the mean density of the earth is about five times greater than that of water, and consequently, about double that of the mineral crust of our globe*.” Laplace considered the mean density of our spheroid as = 1°55, the solid surface being 1. Accord- ing to Baily, the density of the earth is 3°9326 times greater than that of the sun, and is to that of water as 11 to 2+. Superfecial Distribution of Land and Water. The relative proportion of dry land to the ocean, as it at present exists, is such, that nearly three-fourths of the whole surface of the globe may be assigned to the latter. Of the former, the configuration is very various, presenting the greatest surface in the Northern hemisphere. Although the land sometimes rises high above the level of the sea, according to our general ideas on such subjects, it is, in reality, but slightly removed above that level, when considered, as it should be, with reference to the radius of the earth{. The superficies of the Pacific Ocean alone is estimated as some- what greater than that of the whole dry land with which we are acquainted. Dry land can only be considered as so much of the rough surface of our globe as may happen, for the time, to be above the level of the waters, beneath which it may again disappear, as it has done at different previous periods. La- place calculated that the mean depth of the ocean was a small fraction of twenty-five miles, the difference produced in the diameters of the earth by the flattening of the poles. It has been variously estimated at between two and three miles. ‘The mean heightof the dry land-above the ocean-level does not exceed two miles, but probably falls far short of it; therefore, assuming two miles for the mean depth of the-ocean, the waters occupying three-fourths of the earth’s surface, the present dry land might be distributed over the bottom ofthe ocean, in such a manner that the surface of the globe would present a mass of waters ; —an important possibility, for, with it at command, every va- riety of the superficial distribution of land and water may be imagined, and consequently every variety of organic life, each suited to the various situations and climates under which it would be placed. The surface of the globe’s solid crust is so uneven, that the ocean, preserving a general level, enters among the dry land * Traité de Géognosie, ed. 2me, tom. i. p. 18. + Baily, Astronomical Tables. t See the diagram in my Sections and Views illustrative of Geological Phenomena, pl. 40.
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