SANDSTONE PETRIFACTIONS. 159 



not pretend to say, whether the great African De- 

 sart is of the same formation with that in which the 

 petrifactions I have been speaking of are found ; bnt 

 it appears from travellers, particularly Mr Home- 

 man, that this immense sandy deposit is, in many 

 places, full of innumerable fragments of petrified 

 wood. Trunks of trees IS feet in girth are men- 

 tioned : and Horn cm an says, that though these pe- 

 trifactions are generally black, yet, in some cases, 

 they are ash-grey, and resemble natural wood so en- 

 tirely, that they are sometimes brought in by mis- 

 take for fuel. 



By what process of Nature does the wood, on 

 such occasions, disappear, and particles of quartz, 

 cemented by clay, or marl, or other quartz, take its 

 place ? This appears to me to be a matter by no 

 means easily accounted for, on any principles at 

 present recognized. And the difficulty is augment- 

 ed in a very high degree, when we adopt the com- 

 monly received doctrine of the mechanical forma- 

 tion of sandstone. For how can we conceive, that 

 small pieces of quartz, so large, however, as to be 

 sufficiently discernible by the naked eye, should 

 make their way into the densest parts of an oak 

 tree, and in time so completely displace all its 

 woody fibres, as that nothing of these should ap- 

 pear, but the whole become a uniform mass of these 

 little pieces of quartz, connected together as before 

 stated ? We cannot admit, certainly, that a parti- 

 cle of sand, or piece of quartz, so large as to be vi- 



