434 



This is particularly obvious in the specimen represented at PL 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 if 

 they had been slightly touched with a brush dipped in light brown 

 bitumen. In the other specimens, just mentioned, the appearances 

 are 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 the 

 specimen No. 4, the sand-stone is coloured brown beneath the 

 surface, as if by the penetration of the fluid bitumen, which the 

 loose gritty texture of the stone would doubtlessly have readily 

 admitted. 



The iron-stone nodule, on being split, affords the most satisfac- 

 tory evidence as to the nature of the change which the vegetable 

 matter undergoes in these cases, since here, bitumen will uniformly 

 be found to have taken the place, which vegetable matter had ori- 

 ginally possessed. Reverting, therefore, only to the position, that 

 vegetable matter, secluded from the air, in a moist situation, will 

 pass through a certain fermentative process, by which it will be 

 ^converted to bitumen ; the key to this enigmatic phenomenon is at 

 once found. The leaf, involved in the tenacious argillaceous mat- 

 ter, necessarily forms a mould bearing its exact form ; and after a 

 certain period, during which the surrounding mass acquires a greater 

 degree of hardness, and a nodular form, the vegetable matter 

 changes into bitumen, which fills the mould, and assumes exactly 

 the same form which the leaf originally bore. If, therefore, the 

 nodule be now split, one of these two circumstances will occur* 

 either the bitumen will, by the breaking of the nodule, be separated 

 and lost, leaving the impressions on both sides of the leaf perfect: 

 or, as is most commonly the case, it will separate from one side 

 only, and adhere to the other; when the side from which it has 

 separated will yield the impression of the leaf, and the bituminous 

 matter itself, possessing the place of the leaf, will present a surface 



