74 THE STORY OF THE EARTH. 



CHAPTER IX. 



THK CLASSIFICATION OF w \ rER-FORMED ROCKS. 



In every country breaks exist in the con- 

 tinuity of the strata. Such interruptions in 



que nee are in pi at the present day by tin- 



upheaval of land of existing islands, and conti- 

 nents, which intermits the deposition of marine 

 strata. Such breaks are evidenced by want of 

 conformity in the order of succession ni the de- 

 ts. This is one of the main grounds for di- 

 viding geological deposits from each other. The 

 breaks which exist in any one country are some- 

 what limited in the area which they affect. They 

 Can never be world-wide divisions between strata. 

 Strata are also divided according to their differ- 



es in predominant mineral character. The 



changes which take place in the prevalent types 

 of extinct life winch they severally preserve, give 



inds f<>r division of deposits, so that the ca- 

 tion of an ancient group of animals, or the in- 

 coming of a new group, makes a division possible 

 between the strata. 



- no necessary connection between the 



k in stratification, termed unconformity, and 



the break in life I recjiiently there is a great 



change in fossils between two successive 



without an indication of unconformity. [t 

 difficult to understand how the life- can 



iblv without a change in the level "t 



I land, w Inch < aiiM-s the I ;ii adja- 



•<•. An un< A of 



evidence of an unrepre- 

 • ime I han < on^ormil 



