GENERAL PRINCIPLES. 
109 
hills and valleys as before, to be again invaded by the sea and buried 
under additional accumulations of sediments. 
But these animals and plants will not be the same as those which occu- 
pied the same space formerly and have left their fossil remains imbedded 
in the previous deposits, laid down ages before ; neither will the marine 
forms which the next encroachment of sea brings, be a repetition of the 
species whose remains were deposited in its former advance. Everything 
will show the marks of change. Old forms of life will have passed away 
and new ones occupied their places. And these different accumulations 
will be distinguishable, one from another, both by the breaks of continui- 
ty which separate them, being in geological language, unconformable, and 
also by their different fossil contents ; being thus grouped into formations 
and systems. 
And furthermore, an extensive comparison of observations made in all 
quarters of the globe has shown that the life forms, whose successive de- 
velopments have left their impress upon the rocks, have not only under- 
gone great changes from age to age, from formation to formation, but that 
these changes have been in certain determinate directions, — involve more 
than mere change, and indicate also progress; and the steps of this pro- 
gress every where follow the same order, so that, the order of succession 
of different formations having been made out, in one part of the world, 
the same order is found to hold in every other part. 
It is not meant that the entire series is completely represented every 
where; which indeed could not happen, inasmuch as sediments are de- 
posited only over those portions of the earth which are submerged, and 
not on the land surface existing at the same time; on the contrary, some 
of its terms are generally wanting, and any number of them may be ; but 
those terms which exist, preserve every where the same relation to each 
other of upper and lower, newer and older, and observe the same order 
of progression of living forms, the study of whose relations and succes- 
sions in their ancient types, in the lower formations, is styled Paleon- 
tology. 
The geological . chart which follows, will illustrate to the eye, in one 
view, these several points. It represents graphically the relations of su- 
perposition, succession, relative age, and shows the sub-divisions and the 
relations of the different series in different regions, to one another and to 
the complete, typical series. 
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