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 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 matter, 
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, pos- 
sessing the place of the leaf, will present a surface similar to that 
