361 



depending on lie degree of its impregnation with earthy particles, 

 are of course variable and indeterminate. If exposed to tr"> 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 con- 

 sumed. 



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 respect- 

 ing 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 

 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. 







VOL. I. 3 A 



