10 THE DEVONIAN AND CARBONIFEROUS. [bull 80. 



The present essay employs the historical method alike in the summari- 

 zation of present knowledge and in the formulation of the principles of 

 geologic correlation. It groups facts and opinions as to Carboniferous 

 and Devonian formations about certain specific problems of correlation, 

 and traces the history of the discussion of each problem. In connec- 

 tion with the historical summaries there is much incidental discussion 

 of the principles of correlation, and they are afterward classified in a 

 closing chapter. The author concludes, from the American standpoint, 

 that in a universal classification of formations it is not practicable to 

 employ classific units smaller than periods. 



Very respectfully, your obedient servant, 



G. K. Gilbert, 

 Geologist in Charge. 

 Hon. J. W. Powell, 



Director U. JS. Geological Survey, 



