These are deposits formed through solution by two distinct processes viz.

(1) Chemical.

(2) Organic.

1. Siliceous deposits of chemical origin:

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Quartz is insoluble but other forms of a silica are fairly soluble in natural alkaline waters but essentially nearly all silica is transported in colloidal rather than true solution.

Siliceous sinter:

In volcanic region, sometimes hot springs bring up silica and deposit it in the mounds and terraces about the orifices of eruption. This material consists of cryptocrystalline or opaline silica. The deposition is mainly due to evaporation and cooling of waters but certain algae and organisms also initiate precipitation of silica.

Chert and flint:

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They occur as irregular nodules or tabular masses, which consist of minutely-crystalline or cryptocrystalline silica. Most cherts were deposited directly from the sea water replacing carbonate rocks with which they are closely associated and frequently interbedded.

2, Organic siliceous deposit:

They result from direct precipitation by organisms which extract silica as opal from natural waters and bodily contribute to the siliceous deposit. Radiolarians, diatoms, silica’ Sponges are the organisms involved in the deposition of silica.

(a) Radiolarian oozes:

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These are a group of single-celled animals which construct skeletons of silica, which owing to their relative insolubility can sink to greater depths and form radiolarian ooze. The radiolarian ooze are closely associated with the ‘Abyssal- red clay’.

(b) Diatoms are microscopic plants, which secret minute, ornamented, spherical and discoidal body composed of silica and which ultimately gives rise to siliceous deposits after their death.

Calcareous Deposits:

Calcareous deposits may be of chemical as well as of organic origin.

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1. Chemical or inorganic-limestone:

They result by precipitation through evaporation, chemical reaction or by other physic chemical means.

(a) Travertine or tufa:

These are limestones formed by the evaporation of spring and stream waters containing calcium caibonate in solution.

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(b) Kankar:

It is a nodular iron-rich calc-sinter, formed by capillary action.

(c) Stalactite and stalagmite (dripstone).

(d) Oolitic limestone.

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2. Organic-calcareous deposits:

(a) Fossiliferous limestone:

These ate formed from the shells of marine animals and also by corals.

(b) Chalk:

It is a fossiliferous limestone composed of the shells of protozoans (single-celled animals, i.e., the foraminifera etc.)

(c) Marl:

This name applies to mixtures of shells and shell fragments with muds and sand. Thus it is an impure limestone.