NATUEE OF FOSSIL PLANTS. 699 



vegetation which has existed in earlier periods of the world's his- 

 tory. Vegetable remains found imbedded in geological formations 

 are called fossil plants ; and the condition in which these fossils 

 occur are exceedingly varied, both as to the nature of the substance 

 preserving the vegetable forms, and the degree of perfection of the 

 forms preserved. 



The principal kinds of fossils may be classed as follows : 1. Petrified 

 plants, in which the structures of plants have been more or less completely 

 impregnated with mineral matter, hardening them into a stony mass. 

 They present various modifications, in which more or less of the organic 

 matter remains, completely impregnated with mineral substances, or 

 where the mineralization is so complete that the organic substance has 

 totally disappeared. The mineral substance of such fossils is different 

 in different cases. Silicified remains are the most common ; fossils im- 

 pregnated with carbonate or sulphate of lime abound in other strata, 

 while fossils of dense or earthy ironstone, argillaceous ironstone, and, 

 lastly, iron-pyrites are frequent in particular rocks ; impregnations with 

 rock-salt, oxide of copper, alumina, &c. are rarer. 



2. Coal, where the vegetable substance is more or less completely con- 

 verted into a solid, black, combustible carbonaceous substance, of stone- 

 like aspect. This occurs in almost every possible modification, in masses 

 or in the form of isolated plants or organs of plants, from the solid stony 

 anthracite to the brown coal or lignite, which preserves the organic texture 

 and is recognizable at first sight as vegetable matter. Coal-beds are 

 formed through the accumulation of vast masses of vegetation, and their 

 conversion through pressure and chemical changes into solid masses ; but 

 leaves, steins, or parts of stems, such as layers of bark, fruits, &c., con- 

 verted into coal, are found isolated in strata of various composition. 

 With these last are intimately connected the numerous fossils which are 

 true petrifactions, but have the organic matter preserved in the mineral 

 substance in the condition of coal, giving a coal-like aspect to the fossil. 



3. Impressions or natural casts of plants or organs of plants, which have 

 been formed by the vegetable objects being incrusted by, or imbedded in, 

 mineral substance and decaying subsequently to the solidification of the 

 enclosing substance ; the cavity left by the decayed vegetable may be 

 filled up by the same or a different mineral substance ; and casts of the 

 internal parts of stems &c. are met with, from the penetration of the 

 mineral matter into cavities formed by the quicker decay of succulent 

 structures, such as pith. 



4. Objects contained in Amber, the fossil resin of a Pine, which has 

 accidentally enclosed various vegetable and animal bodies which it flowed 

 over while" liquid. The objects are sometimes thoroughly impregnated 

 with amber, like microscopic objects enclosed in Canada balsam, these 

 having been enclosed in a dead or dry condition ; in other cases, where 

 fresh organs have been enclosed, hollow casts only are found, the enclosed 

 matter having been more or less decomposed. 



The study of vegetable fossils is far less satisfactory than that of animal 

 remains, since, in the great majority of cases, the structures most dis- 

 tinctive of the subordinate groups of plants are formed of very perishable 

 matter. Genera, and even species, of animals may be recognized by bones 



