88 HAND A. GEOLOGY. 



by their annual fall, cause the sea to retire before 

 them. 



It will presently be seen in the general account of 

 this series, that the same character pervades it in the 

 interior. To have rendered that sketch more perfect, 

 a map of the continental districts where it occurs would 

 have been required. That map must be reserved to a 

 future period; but to have omitted all notice of these, 

 would have been to leave imperfect a subject of which 

 the interest chiefly depends on the extent and general 

 characters of the objects which it comprises. It is con- 

 venient to describe insulated portions of the surface, 

 but it is necessary for geological science to trace their 

 affinities to each other and to the general structure of 

 the earth. 



