434

This is particularly obvious in the specimen represented at PI . III.Fig. 5; in which, on examining it by aid of a magnifying glass,the projecting particles of the sand-stone will be seen tinged, as ifthey had been slightly touched with a brush dipped in light brownbitumen. In the other specimens, just mentioned, the appearancesare similar, except as to the difference of colour, which is some-times so dark as nearly to approach, as in Fig. 3, to black. In thespecimen No. 4, the sand-stone is coloured brown beneath thesurface, as if by the penetration of the fluid bitumen, which theloose gritty texture of the stone would doubtlessly have readilyadmitted.

The iron-stone nodule, on being split, affords the most satisfac-tory evidence as to the nature of the change which the vegetablematter undergoes in these cases, since here, bitumen will uniformlybe found to have taken the place, which vegetable matter had ori-ginally possessed* Reverting, therefore, only to the position, thatvegetable matter, secluded from the air, in a moist situation, willpass through a certain fermentative process, by which it will beconverted to bitumen; the key to this enigmatic phenomenon is atonce found. The leaf, involved in the tenacious argillaceous mat-ter, necessarily forms a mould bearing its exact form; and after acertain period, during which the surrounding mass acquires a greaterdegree of hardness, and a nodular form, the vegetable matterchanges into bitumen, which fills the mould, and assumes exactlythe same form which the leaf originally bore. If, therefore, thenodule be now split, one of these two circumstances will occur—either the bitumen will, by the breaking of the nodule, be separatedand lost, leaving the impressions on both sides of the leaf perfect:or, as is most commonly the case, it will separate from one sideonly, and adhere to the other; when the side from which it hasseparated will yield the impression of the leaf, and the bituminousmatter itself, possessing the place of the leaf, will present a surface