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sitions in such a manner as to ascertain what volatile matters enter 
into their composition, would, perhaps, sometimes evince the unex- 
pected presence of bitumen both in horn-stone and flint. 
Yours, &c. 
LETTER XXXVIII. 
CALCAREOUS WOOD LIMB FREQUENTLY HELD IN SOLUTION, IN 
WATER VARIOUS FORMS OF DEPOSITION CONFETTO EJ TIVOLI 
QUARRIES OF TUFACEOUS STONE STALACTITIC CAVERNS. 
The mineralized wood, of which we shall treat in this Letter, is 
of such a nature as will not oppress us with such difficulties, whilst 
endeavouring to ascertain its mode of formation, or the nature of 
its constituent parts, as were opposed to our inquiries, respecting 
the same subjects, whilst examining silicious wood ; since the nature 
of the mineralizing matter is such as to allow of its removal by such 
agents as will not act so destructively on the vegetable matter, which 
forms the substratum of the fossil. 
Calcareous wood, wood impregnated with lime, is neither so 
hard nor so heavy, as silicious wood; it may be scratched by a 
knife even in its hardest parts; and in some parts will allow of 
being cut, almost like recent wood; its hardness and its gravity, 
