IOWA ACADEMY OF SCIP^NCES. 33 



valley as that lueutioued as lying jnst east of Buffalo bridge, Madison 

 county, or of those in White Oak township, Warren county. 



A similar statement may be made in regard to the river valleys. The 

 rivers wander here and there over a partly alluvial plain with drift along 

 the margins, at times even on the very banks of the rivers themselves. 



Comparing these different data it is clear the river valleys were mai'ket! 

 out chietly in pre-glacial times. During Mesozoic and Tertiary times when 

 this region was subject to constant erosion, wide valleys were cut into the 

 carboniferous strata as deep as the present valleys. While the drift is an 

 important factor in the present configuration of the country, yet in the 

 region referred to the ice had little to do in erosion, and the waters from 

 melting ice sought in general the natural previously determined drainage 

 courses thus keeping open the rivers and many of the chief ravines of pre- 

 glacial times, while only the lesser ravines have been marked out since the 

 drift was deposited. 



STRUCTURE OF THE MYSTIC COAL BASIN. 



BY H. FOSTER BAIN, IOWA GEOLOGICAL SURVEY. 



The lower measures of the Iowa-Missouri coal field consist of a series of 

 sandstones, shales, fire clays and coal beds, which have been found to inter- 

 lock in a characteristically irregular manner. The different individual beds 

 have, with rare exceptions, only a limited extent, and frequently grade into 

 each other in a manner making their stratigraphy quite complex. This varia- 

 bility has been recognized by many workers* and has recently been elab- 

 oratedf so fully that only a reference is necessary in this connection. 



The explanation of the irregularity is found in the conditions of the 

 depositions of the beds. It depends primarily upon the facts indicated so 

 abundantly by the nature of the beds themselves— that these measures are 

 marginal depositions, and it has been suggested^ that in this field the lower 

 coal measures represent the marginal deposits, of which the upper coal 

 measures are the, in part, contemporaneous open sea beds. 



In certain portions of the field the irregularities may be directly traced 

 to the iuffuence of the uneven nature of the fioor upon which the beds were 

 laid down. 



•Swallow: Rep. Mo. Geol. Sur., p. 87, .Jefferson City, 1853. 

 Wortlien: Geol. of Iowa, vol. I, p. 250. 1858. 



Broadhead: Rep. Mo. Geol. Sur., II., p. 1H6. Jefferson City, 1872. 

 Norwood: liep. Mo. Geol. Sur., pp. 200-215. 1873-1874. Jefferson City, 1874. 

 tlveyes: Stratigraphy of the Carboniferous in Central Iowa; Bui. Geol. So. Am II 

 pp. 277-292, 1891. 



Winslow: Mo. Geol. Sur., Prelim. Rep. on Coal, pp. 21-22, 1891. 



tWlnslow: Missouri Coal Measures and the Conditions their Deposition; Bui 

 Geol. Soc. Am., III., 109-121, 1892. 



Keyes: Geol. Sur. Iowa, vol. I., First Ann. Rep., pp. 84 85, Des Moines, 1893. 

 3 



