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, together, a 
considerable space of ground*. 
Yours, &c. 
LETTER VIII. 
CONSTITUENTS OF VEGETABUES 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 
geology, 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 capital of 
the kingdom of Fezzan, in Africa, in the years 1797-8. 
