75 



but, in some instances, it is of a light grey, and then so much re- 

 sembles wood in its natural state, that the slaves would often collect 

 and bring it in for the purposes of firing. 



These petrifactions are, sometimes, scattered in single pieces ; 

 but are oftener found in irregular layers, or strata, covering, toge- 

 ther, a considerable space of ground*. 



Yours, &c. 



LETTER VIII. 



CONSTITUENTS OF VEGETABLES.. ..FOOD OF VEGETABLES. 



HAVING, in my former letters, pointed out some of the numerous 

 places, in different parts of the earth, where fossil vegetables have 

 been found; and having thereby enabled you to judge, how very 

 generally they are diffused, beneath the surface of the earth, I shall 

 now endeavour to describe the various states into which vegetable 

 matter is reduced, after ceasing to perform the functions of vege- 

 table life. 



As in doing this, I am aware that I shall be under the necessity 

 of disposing, amongst altered vegetable matter, some substances, 

 which those, whom I, and every other student in chemistry and ge- 

 ology, must regard as high and respectable authorities, have denied 

 the propriety of thus classing, such an arrangement shall be 

 adopted, as appears to be best calculated to place the opinions I 



* The Journal of Frederic Horneman's Travels from Cairo to Mourzouk, the capi- 

 tal of the kingdom of Fezzan, in Africa, in the years 1797-8. 



