On the basis of the method of preservation, fossils are grouped into eight main categories and comments on fossil spots of these categories are mentioned below:
Category # 1. Compression (e.g., Zamites sp.):
1. Parts or fragments of plant are buried beneath the sediments in this fossil.
2. The buried specimen becomes flat. It is retained in the form of a carbonaceous film, and the outlines of the external features become clearly demarcated (Fig. 12).
3. Major parts of the organic matter of the specimen in this type of fossils are preserved in the form of impressions.
4. On the surface, it is retained in the form of a carbon layer without any definite shape.
5. Details of the cellular organization of the fossilized material are not visible in compressions.
Category # 2. Impression (e.g., Zamites Sp.):
1. This type of fossil shows true replica of the original plant material.
2. Entire plant or plant parts are preserved within the layers of the earth (Fig. 13).
3. Organic matter of the plant materials are absent in this type of fossil.
4. Details of the cellular organization are also not visible in the specimen.
Category # 3. Petrifaction:
1. In this type of fossils (Fig. 14) the original plant material is replaced molecule by molecule by certain minerals.
2. The replacing minerals chiefly belong to silica, or silicates, sulphates, carbonates and phosphates of magnesium and/or calcium.
3. During the process of fossilization, the minerals infiltrate into the plant tissue in solid form, mainly because of the interaction between the compounds and the minerals released from the disintegrating cell walls.
4. Exact cellular organization of the original plant material is seen in such fossils.
Category # 4. Mold, Incrustation and Cast:
1. When hard sediment deposits around the plant material, it forms mold.
2. In uncompressible material (e.g., sand stone or iron stone), the external mold of a plant is called incrustation.
3. When crystalline substances, such as iron pyrites, chalerite, etc., are deposited into the plant parts, it results in the formation of casts.
4. Around such casts, the carbonates form a protective covering upon cooling (Fig. 15).
5. External structure of original parts of plants are shown by casts.
6. The original organic matter of plant parts is absent in casts.
7. The cellular details of plant parts are also absent in casts.
Category # 5. Peat (Compactions):
1. Peat is actually a kind of litter (dead plant or animal material) layer found in some very wet or waterlogged habitats, such as bogs, which decompose very slowly.
2. From the geological point of view, peat is regarded as the youngest member of the series of coals of different ranks, including brown coal, lignite and bituminous coal.
3. Peat is used as fuel after being air dried.
4. Peat is low in ash but contains a high percentage of moisture.
Category # 6. Coal Balls:
1. A coal ball (Fig. 16) is a calcareous nodule, usually containing large amount of petrified plant remains.
2. The size of the coal balls ranges from a few centimeters to several metres.
3. In some wet habitats, such as swamps or bogs, the plant debris in-filter with carbonates of calcium and magnesium, etc. which restrict their conversion into coal, and this results into petrifaction.
4. Coal balls are found in some seams of coal.
5. Coal actually comprises the fossilized remains of plants that lived in former geological periods.
6. The original plant tissue has been more fully decomposed and converted into carbon.
Category # 7. Amber:
1. It is a hard, pale yellow, and sometimes reddish or brownish fossil resin of extinct pine trees, such as Pinus succinifera.
2. It is an exceedingly hard and brittle substance.
3. It occurs in several forms, the most important of which is succinite.
4. Some of the forms of amber are also transparent or almost opaque.
5. Amber has long been in use in the world since last thousands of years for ornamental pieces. It is widely used for beads and other ornamental purposes, and is often carved.
6. It is also used for mouthpieces of pipes and holders for cigars and cigarettes.
Category # 8. Petroleum (Oil):
1. It is believed that petroleum had an organic origin and was formed under pressure from the minute floating plant and animal life of former shallow seas.
2. It is an oily, dark-coloured, flammable liquid.
3. It is a form of bitumen or a mixture of various hydrocarbons, occurring naturally in the upper strata of the earth in various parts of the earth.
4. It is commonly obtained by drilling.
5. It is used in its natural state or after treatment as a fuel, or separated by distillation into various products such as kerosene, paraffin, benzine, naphtha, or gasolene.