I 134 ] 



for silicification than others. A good drainage of 

 the percolating waters appears necessary, so that 

 when that medium has deposited the silex, which 

 it holds in solution, it may readily run ofF and 

 make way for a fresh supply. Gonsequently the 

 greatest cruanti ties are found lying above the 

 more compact tuff and covered by or enveloped 

 in the loose, porous, superficial earth or volcanic 

 sands, and many of these emit fire when struck 

 by iron. On the contrary such wood, as has been 

 entangled in the more compact and solid tnfFs, 

 still remains soft and may be readily cut away 

 with a knife and, when fresh, presents every ap- 

 pearance of coramon charcoal saturated with wa- 

 ter and easily crumbles in the hand ; these how- 

 ever, when preserveddry for a few months, attain 

 considerable hardness and then offer eifectual 

 resistance to the knife, which extracted them 

 from their original position ; cases have been 

 observed of the two varieties being almost in 

 contact, when the one imbedded in the close tuff 

 was soft and friable, whereas its neighbour, that 

 merely reposed on that formation, had become 

 perfecty hard and solid. 



All these woods may have been originally car- 

 bonized by volcanic action and thus preserved 

 from putrefaction ; this supposition will account 

 for the logs of stone of 10 to 12 feet in length 

 Leing found cracked across the grain, in irregu- 

 iar lengths, yet still lying in the same continuous 

 straight line. A mass of charcoal would easily 

 givc way under an increase of pressure; but that 



