GEOLOGICAL ENQUIRY. 37 
fied by large admixtures of animal and vegetable 
remains. 
As the action of all these forces will be ren- 
dered most intelligible by examples of their 
effects, I at once refer my readers for a synoptic 
view of them, to the section Avhich forms the first 
of my series of plates.* The object of this sec- 
tion is, first, to represent the order in which 
the successive series of stratified formations are 
piled on one another, almost like courses of 
masonry ; secondly, to mark the changes that 
occur in their mineral and mechanical con- 
dition ; thirdly, to show the manner in which all 
stratified rocks have at various periods been 
disturbed, by the intrusion of unstratified crys- 
talline rocks ; and variously affected by eleva- 
tions, depressions, fractures, and dislocations ; 
fourthly, to give examples of the alterations in 
the forms of animal and vegetable life, that have 
accompanied these changes of the mineral con- 
ditions of the earth. 
From the above section it appears that there 
are eight distinct varieties of the crystalline un- 
stratified rocks, and twenty-eight well defined 
divisions of the stratified formations. Taking 
the average maximum thickness of each of these 
divisions, at one thousand feet,t we should have 
* 
The detailed explanation of this section is given in the de- 
scription of the plates in vol. ii. 
t Many formations greatly exceed, whilst others fall short, of 
the average here taken. 
