80 GEOLOGICAL FORMATIONS. 



though the exact line of separation is not always clearly dis- 

 tinguishable. 



In Missouri, however, it has been customary to regard the 

 Goal Measures as made up of three distinct members : Lower, 

 Middle and Upper. These three divisions, though repeatedly 

 described in this and the adjoining states, have never been 

 clearly contrasted, while in the field all tests for distinguish- 

 ing sharply the several divisions utterly fail. Furthermore the 

 similarly named subdivisions of one state do not at all corre- 

 spond with those of a neighboring district. 



The early impression of this fact in the recent considera- 

 tion of the coal-field lying to the north of Missouri, quickly 

 led to the query whether the triple subdivision of the series 

 was in reality a natural one. As investigation progressed, it 

 was soon found that the commonly conceived notion of the 

 classification of the Coal Measures was erroneous ; that any 

 proposal for such an arrangement must necessarily rest upon 

 a highly artificial basis, with no practical value in applying it 

 to the Mississippi valley region. 



In seeking for criteria upon which to establish a natural 

 systematic arrangement of the deposits belonging to the Upper 

 Carboniferous, it was at once discovered that a reconstruction 

 of the original conditions of deposition would aid very mate- 

 rially in the solution of the problem. It had long been a note- 

 worthy fact that the most productive coal deposits in both 

 Iowa and Missouri were situated along the eastern border of 

 the Carboniferous area, and that the western part of the area 

 was occupied chiefly by limestones and calcareous shales. 

 Detailed cross-sections of the geological formations, carefully 

 constructed, showed that the coal-field of the two states men- 

 tioned formed a part of what was originally a broad, shallow 

 bay opening to the westward into the great sea which then 

 occupied the greater part of the continent. 



In considering, then, the Coal Measures as a whole, two tol- 

 erably distinct classes of sediments ^were readily recognized : 

 (1) the marginal or coastal deposits, and (2) the beds laid down 

 in the more open sea. 



