642 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 

 see^-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, silex, and iron, which 

 various causes precipitate, and substances exposed to their action for some time become 

 incrusted and premeated 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 appears that fern leaves placed carefully in clay exposed to a red heat, will be 

 made to resemble the petrified plants found in the rocks. 



Travelling from the metropolis across our own island in a north-west direction, we 

 successively meet at the surface with the several groups of strata which appear in the 

 synopsis. Around London we find the tertiary deposits, which spread from thence over 

 nearly the whole of the eastern coast of England to beyond the Humber, covered with 

 various superficial accumulations. The chalk is encountered in the counties of Hertford, 

 Bedford, or Buckingham ; the oolite in Northamptonshire ; the new red sandstone and 

 carboniferous systems in Leicestershire and Staffordshire ; the old red sandstones and 

 Silurians in Salop ; and lastly, the clay-slate, mica-schist, and gneiss groups occur in the 

 alpine or mountainous districts of Great Britain, which extend, with some interruptions, 

 along its western side. In describing the several systems of strata, some writers traverse 

 the series descendingly, commencing with the uppermost or most recent deposits ; but the 

 more natural plan is to proceed ascendingly, beginning with the lowest formations, the 

 earliest in point of time, and the storehouse whose materials have contributed to the 

 production of the rest. The first step in an arrangement of this kind involves a reference 

 to formations of more ancient date than the oldest of the strata the unstratified 

 foundation rock the granitic platform, upon which all the other deposits are based, 

 sinking to depths which the eye of man can never explore, and rising in mountain walls 

 and pinnacles to heights which only a few of his race have reached. This will form the 

 subject of the following Chapter, in connection with the other igneous masses in general, 

 except those which have been formed dumng the age of man, which will hereafter be 

 noticed. 



