Читать книгу Coal-Fired Power Generation Handbook - James Speight G., James G. Speight - Страница 34

Coals have at various times been classified according to the geological age in which they were believed to have originated.

For example, coal paleobotanists have noted that three major classes of plants are recognizable in coal: coniferous plants, ferns, and lycopods. Furthermore, these plant types are not usually mixed in a random manner in a particular coal, but it has been observed that one particular class of these plant types usually predominates in a coal bed or seam. Thus, because of the changes in character and predominant types of vegetation during the 200 million years or so of the coal-forming period in the history of the Earth, it has often been found convenient and, perhaps, necessary to classify coal according to the age in which the deposit was laid down (Speight, 2013).

It should be noted, however, that deposits of vegetable matter are not limited to any particular era or period, but while these deposits occur even in pre-Cambrian rocks, the plants (i.e., terrestrial plants) that were eventually to become coal were not sufficiently abundant until the Devonian period and it appears that such deposits really became significant during the Carboniferous period.