I 86 THE JOURNAL OF GEOLOGY. 



in Ohio and Pennsylvania, as the mountains are approached, the 

 structure increases rapidly in complexity, until in the highly 

 folded and faulted districts attempts to follow out the original 

 state of things may become utterly hopeless. 



The majority of the larger coal deposits of the Western 

 Interior field may be considered then as having been formed in 

 swamps skirting a great shallow gulf, the extent of the produc- 



FlG. 4. Coal Horizon viewed at right angles to shore line. 



five portions of the different horizons being in a measure depend- 

 ent upon the length of time the physical conditions were favora- 

 ble to coal formation. Many short minor episodes doubtless 

 existed between the larger ones, during which comparatively 

 small accumulations of vegetable material took place. 



Another fact to be taken into consideration is that all the 

 coal of the region was not formed in marine swamps, but that 

 some of the minor basins were doubtless originally a very con- 

 siderable distance from the sea, while certain others were formed 

 where open sea conditions prevailed largely. A few seams also 

 appear to have been formed as drift materials in estuaries at the 

 mouths of streams. 



Charles Rollin Keyes. 



