444 
circumstances which imply their rarity. Thus Scheuchzer has 
No. 80j A^rundinis vallatorioe radioed. Luid. p. 180 ; but a reference 
to the work of Lhwydd is fruitless, since its number of pages is 
less than the number referred to ; nor do I discover the descrip- 
tion of such a specimen. Langius presents us with a figure of a 
Rhizolithus, of which he thus speaks : “ suhlutei est colons, et ra- 
dice 7 n pastinacce tenuifolice saiivee radice lutea. Cctsp. Bauhin. Pin. 
preesefert, allatus est e Toggio\.” But that the substance he thus 
describes was merely a figured stone, which had by some accidental 
occurrence assumed the form of a piece of carrot, is most probable : 
for no fact in the history of these substances is more true than the 
position laid down by Wallerius, when speaking of the requisites 
to be possessed by substances to render them capable of becoming 
the subjects of petrifaction. ‘‘ Ea,” he says, “ sit indole et duritia 
ut non facile putrefactionem, vel aliam destructionem, subire pos- 
sit, sed diutius in locis subterraneis incorruptibile conservari.” 
Linngeus also says:J;, “ Corpora petrificanda sint solida : testae ossa, 
ligna. Succulenta deliquescunt et corrumpuntur antequam indu- 
rescant lapidosa.” But even a modern author, justly celebrated 
for his knowledge in chemistry and natural philosophy, speaks of 
a petrified potatoe, a Rhizolithus as inadmissible as that of Lang. 
Argenville, Hell wing, and others, have also described Rhizolithi ; 
and the last-mentioned author not only speaks of them, in even 
an agatine state, but distinguishes them into the roots of dif- 
ferent trees. It must, however, be obvious, that the difference be- 
tween the roots of different trees can hardly ever be such as to 
allow of its being ascertained, to what kind of tree a petrified frag- 
ment may have originally belonged. Thus the specimen repre- 
sented at PI. II. Fig. 2, indubitably deserves to be considered as 
a true Rhizolithus ; since, although it now possesses the hardness, 
* Appendix Herbarii Diluviani, p. 70. f Historia Lapidum Figurat. Helvetiae, p. 54. 
J Systema Naturae, tom. iii. p. 154. 
