640 GEOLOGY. 



epidermis, and of the ligament by which the valves were held together; and bones 

 of reptiles in Tilgate Forest, as light and porous as those of the bear and hyena from 

 the caverns of Germany. On the other hand, fossil remains from the newest tertiary 

 formations are often completely petrified, that is, permeated by, or transmuted into, 

 stone." 



In animal remains, the harder parts are sometimes partially impregnated with mineral 

 matter, while the animal matter is still obvious to inspection ; but far more frequently, the 

 animal matter appears to be almost entirely replaced by the mineral, constituting a genuine 

 petrifaction. Parkinson, however, observes, that probably, in every case, a chemical 

 process would show the presence of considerable organic matter. The mineralising agent 

 is most commonly carbonate of lime, but often silica, or clay, or oxide of iron, and sometimes 

 the ores of metals. Vegetable remains occur, in a similar manner, completely permeated 

 by mineral matter, yet without any destruction of their external character or internal 

 structure, so that the most delicate tissues of plants, their leaves and seed vessels, and even 

 their pollen, are quite distinct. But large masses of vegetation have entirely lost all 

 organic structure, having been bituminised, through that peculiar chemical process which 

 vegetable substances undergo when excluded from the air, and subject to great pressure. 

 Of this process, coal, lignite, amber, and asphalte are examples. There are productions in 

 course of formation at present, strikingly analogous, if not identical, with those changes 

 which the mass of fossil organic remains has experienced. Thus rivers and springs 

 hold in solution a large proportion of lime, silcx, and iron, which various causes precipitate, 

 and substances exposed to their action for some time become incrusted and permeated 

 with the siliceous and calcareous earths, and the metallic oxides. A bone buried in clay 

 containing sulphate of iron, will, in a few years, or even months, have undergone a 

 perceptible change towards petrifaction ; and by an experiment of M. Goppert, it appear* 

 that fern leaves placed carefully in clay exposed to a red heat, will be made to resemble 

 the petrified plants found in the rocks. 



The arrangement of Lehman and Werner, already mentioned, has substantially stood its 

 ground to the present day, and may be more fully stated as follows ; 



Superficial formations. 1. The modern or alluvium, embracing the surface soil, and 

 deposits of sand, clay, and gravel, formed by the ordinary action of water. 2. The 

 ancient or diluvium, deposits of gravel and clay, with boulder stones more or less rounded 

 by attrition, which have been transported from a distance by the extraordinary action of 

 water. Hence the term drift applied to the latter depositions. 



Tertiary. An extensive series of regular strata, marine and fresh-water deposits, 

 alternating, consisting of marls, clays, thin beds of sand, and imperfect limestones, 

 or crag. The remains of animals and plants occur in great abundance, the major part 

 belonging to extinct species. 



Secondary. Vast beds of chalk, clay, argillaceous slate or shale, sandstones, coal, 

 ironstone, and limestone, are constituents of this class, in which vegetable remains are 

 abundant, and the great saurians or lizard-shaped animals. 



Transition. The prevailing rocks are limestones, sandstones, varieties of slate and 

 shale, containing few organic remains, and none of animals of high organisation. 



Primary. Immense beds of hard and compact slaty strata, of crystalline structure, 

 compose this class, in which no organic remains are found. 



As geology has advanced, the tertiary, secondary, transition, and primary classes have 

 each been found to consist of systems of strata, having a distinct character, so as to admit 

 of their being grouped together, and the transition class has been incorporated in the 

 secondary. The diagram on the opposite page gives a proximate view of the principal 

 groups or systems of strata, with their order of superposition. 



