445 
and bears the polish of agate, its external form, as well as its 
structure, sufficiently testifies it to have been originally the root of a 
tree; but of what tree, certainly, no judgment can be formed. 
From the manner in which these bodies are spoken of by the older 
writers on these subjects, it appears to be evident, that they have 
frequently referred to this head merely some of those tufaceous in- 
crustations, which form round pieces of twigs and branches of wood ; 
and which, from their sometimes also resembling the fragment of a 
bone, were generally known by the name of Osteocoll(B. Gesner, 
in his excellent Dissertation on Petrifactions, is obviously of this 
opinion : he says, speaking of Rhizolithi, ‘‘ Hue pertinent petrificata 
ramosa calcaria, arenacea, argillacea corruptarum in terra radicum 
sedem occupantia, Osteocollee et Stelechitee dicta*.” Indeed, so 
much confusion has existed with respect to this substance, that 
many have confounded it with the stalactite, although the great 
and venerable Aldrovandus had, at that early period, fully marked 
the difference. The term Rhizolitlius has also been applied, by some, 
to substances which have obtained their form from the remaining 
earthy particles of roots, and which, being blended with other earth, 
in the mould of the decayed root, have preserved somewhat ot their 
original form ; but, the vegetable structure being entirely destroyed, 
they can only deserve to be considered as the vegetabilia terrijicata 
of Wallerius. 
I have hitherto supposed that bituminization was a process, to 
which all vegetable matter was subjected, under certain circum- 
stances ; and that by this operation it became liable to the influence 
of another process, that of petrifaction. But the absence, m genera , 
of roots from the collections of petrifactions, points out sue 
deviation from this assumed law of nature as seems particuar y 
demand investigation; by which we may, perhaps, be ena e o 
* Joannis Gesneri Tractatus Physicus de .Petrificatis, p. ?1, 
