36 Proceedings of the Ohio State Acadeiny of Science 



today; while alternating with these periods of land extension and 

 often of high relief, were periods wdien the lands were worn to- 

 wards base-level, and widely covered by shallow sea. Periods of 

 emergence have alternated with periods of submergence since 

 very early times ; and certainly since the commencement of the 

 Paleozoic we find no support in geology for a continental evolu- 

 tion at all analogous with what is found in the realm of animal 

 and plant life. 



LARGER PROBLEMS OF GEOLOGY. 



Geology is the history of the earth ; of the whole earth, both 

 shell and core. We are coming to see more and more clearly that 

 the changes which we can see going on today over the earth's 

 surface, and which we can infer from the study of the rocks of 

 the earth's crust, are but expressions of changes which are go- 

 ing on within the great mass of the earth, forever hidden from 

 our si^iht. They are symptomatic of deeper and more funda- 

 mental changes. And our geological theory will never be com- 

 plete until we have clear and correct notions of the history of the 

 earth as a globe, and of the changes which are now going on with- 

 in it. We have not that knowledge today, it may be years in com- 

 ing, and in the mean time our writing on these fundamental 

 problems will be highly speculative. Yet such speculation is a 

 necessary part of our science in each advancing stage. If it is 

 carefully controlled by known geological fact, no harm need 

 come. Along two lines of geological theory important advance 

 has been made in recent time. 



Continents and Mountains. 



The condition of geological thought on the subject of the 

 permanence of continents and ocean basins in the middle of the 

 nineteenth century is well summarized in Tennyson's lines : 



There rolls the deep where grew the tree. 



O Earth ! what changes hast thou seen ! 



There where the long street roars, hath been 

 The stiHness of the central sea. 



