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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 impressioa 



