and newer sedimentary, and the contemporaneous deposits. Their nature, and the effects they have produced, are fully considered in the works to which reference is made.

By an inspection of the geological map of England (Wond. pl. x.), it will be seen, that viewed on a broad scale, the several formations appear on the surface in a chronological order, from the south-east to the north-west. Thus the principal Tertiary deposits are situated in the eastern and south-eastern parts of our island; and proceeding towards the northwest, we successively pass over the Upper Secondary -the Chalk, Oolite, and Lias; then the Lower Secondary the Carboniferous and Devonian groups; next the Silurian; and lastly the Cambrian and Metamorphic rocks. It is this distribution of the strata that has determined the physical character of the geography of England. The Alpine or mountainous districts, which extend north and south along the western portion of England and Wales, from Cornwall to Cumberland, are formed by the elevated masses of the Metamorphic, Cambrian, and Silurian rocks.

Next we have a band of the Carboniferous, and associated strata of red marls and sandstones, with occasional interspersions of Metamorphic and Basaltic masses, stretching from the coast of Devonshire, through the midland counties, by Derbyshire

to Durham. On the east of this line, the Lias, Oolite, and Chalk, constitute a calcareous district, which spreads over the whole of the eastern and south-eastern portion of the island, from the southern shores of Dorset, Hants, and Sussex, to Flamborough-head and Whitby, on the coast of Yorkshire. And lastly, on the Upper Secondary strata in the eastern maritime districts, and in basins or depressions in the south-eastern, the Tertiary formations appear; and on these last are distributed vast local accumulations of Drift, or Alluvial debris.

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CHAPTER III.

ON THE NATURE OF FOSSILS, OR ORGANIC REMAINS.

It is very generally the case, that persons who are not conversant with the nature of organic remains, imagine that all fossils are petrifactions; and unless a specimen has the aspect and hardness of stone, they regard it as of modern origin, and devoid of interest. Hence they are surprised to find among the choicest treasures in the cabinet of the geologist, shells and corals as perfect in form, as if recently collected from the sea-shore; bones as little changed, as if they had been interred but for a short period; and teeth possessing their sharp edges and enamel unimpaired. In my early researches I fell into this error, and threw away many beautiful shells that were associated with ammonites, in the marl at Hamsey, supposing, from their perfect state, that they had been accidentally imbedded, and were not genuine fossils. But the state of preservation, and the degree of change which an organic body has undergone in the mineral kingdom, have no necessary relation to its antiquity. The shells in some of the ancient secondary strata are frequently as

perfect as those of modern tertiary deposits. I have collected in the lowermost clays of the Wealden, fresh-water shells having traces of the 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.

Another prevalent error is that of considering incrustations to be petrifactions; a mistake which is sanctioned, by the custom of calling waters that are loaded with calcareous earth (lime), and deposit it in considerable quantity, petrifying springs or wells; as those of Matlock, and other places in Derbyshire. (Wond. p. 58.) But incrustations are not petrifactions; stems and branches of trees, skulls, bones, shells, &c., are simply invested with a calcareous coating or crust, which is sometimes porous and friable, but often crystalline and compact. The enclosed bodies are not permeated by the stony matter; if the mass be broken, or the incrustation removed, we find either the twig, or stem, dry and shrivelled, as in the specimens, figs. 2, 3, 4, Plate III.; or tubular cavities left by the decay and removal of the vegetable structure, as in fig. 10, Plate III.

But although incrustations be not petrifactions, natural specimens, (not the so-called petrified nests

and wigs, in which the bad taste of the guardians of the Derbyshire springs is embodied, and dispersed all over England,) are objects of considerable interest, as illustrative of a process, by which important changes are effected in the mineral kingdom. Thus springs as clear and sparkling as poets ever feigned or sung, may transform beds of loose sand and gravel into rock, and porous stone into solid marble, and cover extensive tracts of country with layers of concretionary and crystalline limestone. This process is effected in the following manner. Most fresh water holds in solution a certain proportion of carbonate of lime; and changes of temperature, as well as other causes, will occasion this calcareous earth to be in part or wholly precipitated. The fur, as it is called, that lines a kettle or boiler which has been long in use, affords a familiar illustration of this fact. At the temperature of 60° lime is soluble in 700 times its weight of water; and if to the solution a small portion of carbonic acid be added, a carbonate of lime is formed, which is thrown down in an insoluble state. But if the carbonic acid be in such quantity as to supersaturate the lime, it is again rendered soluble in water: it is thus that carbonate of lime, held in solution by an excess of carbonic acid, not in actual combination with the lime, but contained in the water, and acting as a menstruum, is commonly found in all waters. An absorption of carbonic acid, or a loss of that portion which exists in excess, will therefore occa