SIGNIFICANCE OF THK UNCONFORMITY 189 



at many horizons in the Coal Measures. That it signifies an important 

 sequence of events has never heen suHiciently emphasized. That the 

 horizon is really a great hiatus has never been fully considered. That 

 the interval represents a period in the history of the region of much 

 longer duration than it took to form all the Coal Measures above it is a 

 phase of the subject never before suggested. 



It has lately been shown * that the present Ozark uplift is of compar- 

 ative recent date — that is, not older than Tertiary. In considering the 

 region as it was in Carboniferous times, the dome must be neglected and 

 the area regarded as forming a lowland plain, the same as the rest of 

 the region was known to be. This is further indicated by the fact that 

 on the highest parts of the dome remnants of the Coal Measures are 

 still found on the beveled edges of the older strata. 



The oscillation of the Carboniferous shoreline in the upper Missis- 

 sippi valley has already been described in detail.f The evidence goes 

 to show that immediately after the Kaskaskia beds were laid down land 

 existed north of the present Arkansas-Missouri boundar3^ This was a 

 region of profound and prolonged denudation. South of the line men- 

 tioned sedimentation continued. The land waste from the northern 

 district was carried into the southern waters. 



The northern area, after the close of the early Carboniferous period, 

 being an area of denudation, suggests an area to which the waste was 

 carried and deposited. There is also suggested a depositional measure- 

 ment of the erosional period. 



Basal Series of Coal Measures in Arkansas 



thickness and general character 



Heretofore the Coal Measures of Arkansas have been regarded as 

 anomalous. They present an enormous development as compared with 

 the Coal Measures of other parts of the Mississippi valley and even of 

 other portions of North America. 



The thickness of the Coal Measures of the Arkansas valley, as esti- 

 mated by Branner,! is nearly 24,000 feet. If present correlations be 

 correct, the highest of these beds in Arkansas are not much, if any, above 

 the horizon of the Betliany limestone of Missouri and Kansas. For the 

 deposition of such an enormous sequence there must have existed ex- 



* Missouri Geol. Survey, vol. viii, 1895, p. 351. 

 t Iowa Geol. Survey, vol. i, 1893, p. 118. 

 X Am. Jour. Sci. (4), vol ii, 1896, p. 235. 



XXVIII— Bum,. aKoi,. Soc. Am., Voi,. 12, 1900 



