COMPARISONS WITH THE FLOKAS OP EUEOPBAN BASINS. 293 



coal-forming conditions at or below water level. This superficial wash may 

 be developed, supplemented, or replaced by other formations in other coun- 

 ties of the State, but in portions of Henry County, at least, it seems to 

 have been eroded and sometimes entirely cut through to the su])jacent rock 

 before the fire clays or lower coal were deposited on the uneven surface. 



The transgression of the water level during the early Mesocarbonifer- 

 ous time has already been discussed by Broadhead,^ Winslow," and Keyes,^ 

 the State geologists. The evidence of the fossil plants not only corroborates 

 their views in general, but it also fixes the time of the encroachment of the 

 sea on the old coast in the region of Clinton. The paleobotanic criteria 

 indicate that the minimum time represented b}^ the unconformity between 

 the Jordan or Owen coal and the subjacent Eocarboniferous terrane is 

 measured by the period required for the deposition of the Potts^nlle and 

 the Clarion group of the Lower Productive Coal Measures, a series of rocks 

 reaching a thickness of over 1,200 feet in portions of the anthracite regions, 

 and exceeding 2,400 feet in southern West Virginia. 



EELATIOjST of the MISSOURI FLORA TO THOSE OF EITROPEAN BASINS. 



ZONE OF THE FLORA IN THE COAL MEASURES OF GREAT BRITAIN. 



In the preceding pages it has been shown that the coals, resting in 

 places directly on the Lower Carboniferous terranes in Henry County, Mis- 

 souri, are probably of a rather later date than the plants of Mazon Creek 

 and the Mori-is coal in Illinois, and that they are very likely older than the 

 Upper Kittanning coal of Pennsylvania. 



It is customary to consider the flora of Mazon Creek, the plants from 

 which are preponderantly identical with those from Missouri, as tj-^Dically 

 representative of the flora existing immediately at the beginning of Lower 

 Coal Measures time, there being but little difference between the plants 

 from Illinois and those of the bituminous Brookville and Clarion coals in 

 northwestern Pennsylvania, which are probably fully as old as those of the 

 Buck Mountain coal, long since made the boundary between the Potts^dlle 

 series and the Productive Coal Measures of the typical section in the South- 



' Amer. Geol., toI. xir, 1894, pp. 380-388. 



= Bull. Geol. Soc. Amer., vol. iii, 1892, pp. 109-121; Amer. Geol., vol. sv, pp. 81-89; Prelim. Kept. 

 on Coal, Geol. Surv. Missouri, 1891, p. 19. 

 = Amer. Geol., vol. sii, 1893, p. 100. 



