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 in 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. 



State 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 



