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CHAPTER III. 
Different circumstances under which the secondary and tertiary formations may 
have originated— Secondary series formed when the ocean prevailed: Tertiary 
during the conversion of sea into land, and the growth of a continent — Origin 
of interruption iu the sequence of formations — The areas where new deposits 
take place are always varying — Causes which occasion this transference of the 
places of sedimentary deposition — Denudation augments the discordance in 
age of rocks in contact — Unconformability of overlying formations — In what 
manner the shifting of the areas of sedimentary deposition may combine with 
the gradual extinction and introduction of species to produce a series of deposits 
having distinct mineral and organic characters. 
DIFFERENT CIRCUMSTANCES UNDER WHICH THE SECONDARY 
AND TERTIARY FORMATIONS MAY HAVE ORIGINATED. 
We have already glanced at the origin of some of the prin- 
cipal points of difference in the characters of the primary and 
secondary rocks,, and may now briefly consider the relation in 
which the secondary stand to the tertiary, and the causes of 
that succession of tertiary formations described in the last 
chapter. 
It is evident that large parts of Europe were simultaneously 
submerged beneath the sea when different portions of the secon- 
dary series were formed, because we find homogeneous mineral 
masses, including the remains of marine animals, referrible to 
the secondary period, extending over great areas; whereas the 
detached and isolated position of tertiary groups, in basin 
or depressions bounded by secondary and primary rocks,, 
favours the hypothesis of a sea interrupted by extensive tracts 
of dry land. 
Stale of the Surface when the Secondary Strata were formed. 
Let us consider the changes that must be expected to accom- 
pany the gradual conversion of part of the bed of an ocean into 
a continent, and the different characters that might be imparted 
