608

DYNAMICAL GEOLOGY.

Secondly, by temperature and depth of water. The reef-formingcorals grow in the warmer ocean-waters, in which the mean tem-perature for the coldest month does not fall below 68° F. Thelimit in depth appears to depend on the degree of light and press-ure for which the species were made.

The following zones in depth have been recognized by Forbes and other ob-servers for the convenience of marking the distribution of marine species :—

1. The Littoral zone,—or the tract between high and low tide level.

2. The Laminarian zone,—from low water to fifteen fathoms (90 feet). Thiszone is so named from the fucoidal sea-weed, called sometimes Tangle-weed,which is of the genus Laminaria , a plant especially of rocky shores.

3. The Coralline zone,—from 15 to 50 fathoms.

4. The Deep-sea Coral zone,—from 50 to 100 fathoms and beyond.

The zones of oceanic temperature are marked on the Physio-graphic Chart, and are explained on pages 42-44, where also factsare mentioned illustrating the geological bearing of the subject.

2. The nature of different organic products , and the fitness of the speciesaffording them for making fossils and rocks.

(a.) Nature of the organic products contributed to rock-formations. —Thefollowing are some of the general facts relating to the nature of theorganic products contributed by life to the rocks:—

1. Plants afford coal, fossil leaves, and fossil wood.

2. Animal remains are more or less durable according to the pro-portion of stony ingredients present.

3. Shells, corals, and the like contribute to rock-formations almostsolely carbonate of lime, or the material of limestones.

4. Bones, in addition to carbonate of lime, contain much phos-phate of lime and animal matter.

5. Diatoms, Poly cystines, and spicula of Sponges afford silica.

Facts relating to the change of wood to mineral coal are mentioned on page 359.Mineral oil is another result of the decomposition of vegetation. When thecarbon is only sparingly diffused through earth, it gives it a blackish color,which is lost when the material is highly heated.

Plants also afford some sulphur, potash, and soda. Carbonic acid is one ofthe important results of their decomposition.

Some sea-weeds are calcareous like corals, owing to their secreting lime amongtheir sparse, tissues. (See page 67.)

Animal membranes decompose and pass off for the most part as gases. Someof the carbon often remains in the bed in which it is buried, giving it a darkcolor. Impressions of the soft parts of animals, as of Cephalopods, have beenfound in rocks; but they are very rare. The tissues that penetrate shells andbones are sometimes in part retained by the ancient fossil. Two cases are