432 
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Various conjectures have been formed, but none have proved 
successful, in satisfactorily explaining, why the impressions, on 
both halves of the nodule, should represent the same side of 
the leaf. The leaf itself, which formed the impression, being no 
longer to be found, it was expected that it would uniformly 
have left on the mass, in which it was enveloped, the impression 
of its upper, and of its under surface ; but, in by far the greater 
number of instances, this is not the case ; but, on the contrary, 
the one half of one of these split nodules will be found to bear 
the impression of, for instance, the under side of the leaf in basso 
relievo ; the projecting parts of the leaf having produced in its 
impression corresponding depressions ; and on the counterpart of 
the nodule will be found the impression of the same side of the 
leaf in alto relievo ; in the same manner as if the leaf itself, which 
had given the impression, had remained there. M. Jussieu endea- 
voured to explain this curious phenomenon, by supposing that the 
plants or leaves must have floated on water containing muddy 
bituminous particles, which would by degrees deposit on the leaves 
the particles with which it was loaded. By the continued applica- 
tion of these particles, crusts, he supposed, would be formed on the 
leaves, which would bear the figures and impressions of the leaves 
exactly. On the leaves perishing, the crust, thus formed on the 
surface of the leaf, would sink, and, settling on the soft mud, would 
give a copy of the impression which the leaf had formed; which 
would have the exact similar surface, with that which the leaf 
itself would have presented : the bituminous matter with which 
the leaf had been charged, from the water in which it had swam, 
having given to the mould such a surface as would prevent the 
cast from adhering to it. M. Schultz endeavoured to account for 
this circumstance in another way. He supposed that the leaf 
had become involved in a soft earthy matter; and that, as this 
dried, the leaf perished, and left its mould, bearing the impression 
