PRINCIPLES OF STRATIGRAPHIC CORRELATIONS 531 



naturally be made up in greater or smaller part of material derived from 

 the underlying formation. Under average conditions this imitation 

 should be greatest when the subsiding floor is a sandstone, somewhat less 

 when a shale and least when a limestone. In the last the new deposit is 

 usually a calcareous shale or an argillaceous limestone, with the two 

 frequently occurring in alternating layers. 



Of sandstones following older sandstones after a long interval of non- 

 deposition and then grading into some other kind of rock I would cite 

 (1) the case of the Waverlyan Sylamore sandstone in northern Arkan- 

 sas, where it sometimes rests on the Saint Peter and passes upward into 

 black Chattanooga shale, and (2) the apparent gradation from an Ozark- 

 ian sandstone into sandstone and shale of Pottsville age, observed in 

 Missouri near Bolivar and at several localities in the southern part of 

 Saint Clair County. Despite the great time value of the hiatus in these 

 cases it is very difficult to draw the boundary between the old and the 

 younger deposits. 



Similarly obscured interruptions of deposition, with shale under and 

 above the break, are illustrated by ( 1 ) the transition from the black fissile 

 Chattanooga shale to the bluish calcareous Eidgetop shale (early Kin- 

 derhookian), found locally beneath the Fort Payne limestone in middle 

 Tennessee, and (2) the passage from the black Woodford shale to the 

 similar Caney shale when the intervening Sycamore limestone is absent, 

 or to gray sandy shale where the Sycamore is represented in part by 

 shale, or again from these gray shales to the black shales of the Caney, 

 all of which conditions were observed near Wapanucka and in the vicinity 

 of Bengal, in Oklahoma. In the first example the hiatus is small and 

 locally seems to be closed entirely. In the second, as explained on page 

 527, the break, though varying in value, is yet of great importance in 

 each of the three phases mentioned. 



The hiatus in apparently gradual but really broken passages from 

 a limestone to a shale formation is always difficult to establish except 

 between highly fossiliferous beds. As a rule, too, the hiatus is not of 

 great value. The transition from the Trenton to the Utica is often 

 gradual, but in most cases it is possible to show that some interruption 

 of sedimentation occurred between them. The transition from the 

 Chambersburg limestone to the Martinsburg shale in the Cumberland 

 Valley in southern Pennsylvania, Maryland and northern Virginia, like- 

 wise commonly seems gradual. However, as described on pages 323 to 

 328, the two formations are always separated by an hiatus partly meas- 

 ured in places by known absence of hundreds of feet of limestone. 



