37 
fossilia, vidgo dicta, of Linnaeus. But since it is indubitable, that 
the remaining principles have not only entered into entirely new 
combinations ; but that new matter, as carbonic acid, &c. is super- 
added, it must be allowed, that a degree of change, in the nature of 
the substance, must have taken place. Other bodies have been 
considered as secondary fossils, which hardly deserve to be thus 
classed. These are — 1st. Impressions*. These are certain hard, 
but once soft, mineral substances ; which retain the figure which 
has been impressed on them, by some secondary fossil. 2dly. 
CASTsf . These are formed by the deposition of mineral matter in 
the cavities of animal or vegetable substances, or in the vacuities, 
left by the decay of organized bodies, involved in some solid matrix. 
3dly. Incrustations J. These are formed by such an apposition 
of mineral matter generally by precipitation or deposition, as 
effects the envelopeme.it of some vegetable or animal body, in a 
mineral crust. 
Whatever claim to this distinction the two first mentioned of these 
bodies may possess, from the circumstance of their actually owing 
their form to secondary fossils, it can hardly be admitted ; since 
they merely bear the form, and were themselves never endued with 
vegetable or animal organization. With respect to incrustations, the 
last-mentioned of these bodies, it is sufficient to remark, that the cir- 
cumstance of incrustation alone, is not sufficient to constitute a body, 
fossil ; since organic bodies, thus found, are, generally, of but a 
late origin, and, most frequently, unaltered in their structure. Due 
attention will, however, be paid to all these substances, whenever 
they serve to illustrate the actual secondary fossil ; or contribute to 
the explanation of its mode of formation. 
* Impressa, Linnael ; Typolitlii, Wallerii ; Type and Ectype, Breynii ; Figuratornm 
iMpidum Matrices, of others. , 
t Redintegrata, Linnaei ; Petrificata spoliata, Luidii, Noyau, Gallorum. 
t Incrustata, Linnasi. 
