INTRODUCTORY OBSERVATIONS. 
XVll 
well known that granite and its associated rocks 
are, in fact, ancient lavas of various ages ; and 
it is certain that granite has been erupted, even 
since the period when the chalk was deposited. 
The other primary rocks appear, on the contrary, 
to be sedimentary deposits altered by the effects of 
high temperature under great pressure ; such are 
gneiss^ mica slate^ See. 
The secondary rocks contain the fossilised re- 
mains of animals and vegetables, are generally stra- 
tified, and have evidently been deposited by water : 
to this class all the formations of the south-east of 
England belong. These strata, for the convenience 
of study, are subdivided into the secondary^ strictly 
so called, which comprise all the sedimentary rocks 
from the primary to the chalk inclusive : and the 
tertiary, under which division all the beds, from 
the chalk to the alluvial deposits of the modern 
epoch, are placed. 
The organic remains entombed in the sedi- 
mentary strati!, afford conclusive evidence of the 
former existence of a state of animated nature 
widely different to the present ; and furnish data 
by which we can determine the comparative ages 
of the various formations, and even calculate the 
relative periods when the existing momitain chains 
were lifted up. Nay, more ; by these relics, these 
medals, as they have been aptly termed, struck by 
nature to commemorate her revolutions, we learn 
the physical mutations which the surface of the 
earth has undergone, and the temperature of the 
climate of various regions, in periods far beyond all 
human history and tradition ; and, by bringing to 
a 
