361 
^ depending on the degree of its impregnation with earthy particles, 
are of course variable and indeterminate. If exposed to the action 
of the mineral acids, its stony parts are dissolved with effervescence, 
and the vegetable parts are left : by the action of fire the stony 
parts are reduced to pure lime, whilst the vegetable matter is 
consumed. 
As in the silicious, so in this species of fossil wood, it is evident, 
that the lapidifying matter has been deposited from a fluid holding 
it in a state of solution. Pursuing, therefore, the same course as 
was chosen, whilst endeavouring to determine the mode in which 
silicious wood had been formed, we will endeavour to discover the 
source of this impregnating matter. But little difficulty, however, 
presents itself here, since in almost every part of the world, springs 
and rivers exist which contain a considerable proportion of calca- 
reous earth, which is deposited in the form of that species of car- 
bonate of lime, which is termed tufa, on the sides of the channels 
through which these waters flow; and, indeed, on every substance 
they meet with in their current. 
In England the waters of this kind are very numerous, particu- 
larly in the counties of Derby, York, and Somerset; but indeed 
their occurrence is much too frequent to allow of being particu- 
larized ; a few only of the most remarkable circumstances respecting 
them can be noticed. 
Dr. Plot relates* that, in Oxfordshire, there are incrusting waters 
at Somerton ; at North- Ashton, in a field north-west of the church ; 
in the parish of St. Clement, in the suburbs of Oxford ; about a 
quarter of a mile distant, on the right hand of the first way that turns 
eastward, out of Marston-lane. But much better for this purpose, 
he observes, is the water of a pump at the Cross Inn, near Carfax, 
in the city itself. 
* The Natural History of Oxfordshire, p. 34. 
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VOL. I. 
