CHAPTER III. 
Symmetrical Structures of Rocks . 
*1 he fundamental roeks having now been described, it remains to 
determine what changes have happened to them since their deposition. 
The principal effects which are to be traced are due to subterranean 
movements, pervading heat, and molecular attraction. To treat of 
these effects in the order of their occurrence may be difficult, yet 
without some attempt to view the processes in their historical con- 
nection, we are in danger of losing sight altogether of their real and 
necessary dependence. 
The consolidation of stratified rocks is a natural consequence of 
the pressure of the superincumbent masses, whether heat was applied 
or not : yet, since we know that many of the older strata were formed 
under oceans as shallow as those which received the more recent 
deposits, and have not been covered by any great thickness of superior 
rocks, it becomes evident that the higher state of consolidation in 
which we find them must be ascribed to another cause. It is a certain 
truth that the consolidation of stratified rocks is directly in propor- 
tion to their antiquity : for, omitting cases of rocks locally metamorphie 
by contact with igneous masses, we shall find in each of the three 
most abundant stratified rocks — calcareous, — argillaceous, — and arenace- 
ous, a decreasing scale of consolidation, from the primary to the ter- 
tiary systems. 
The argillaceous slates of the primary period are, as to chemical and 
molecular composition, very analogous to the argillaceous beds of the 
mountain limestone system ; but they are more indurated, have more 
