STRATIGRAPHIC CLASSIFICATION STRUCTURAL CRITERIA 465 



broke into pieces in the usual manner, and occasionally one of the frag- 

 ments became separated from its fellows just as they appear in sections 

 of the residual cover of cherty beds today. 



Fossils wholly or partly freed from a limestone matrix by weathering 

 and which, after lodging in hollows of the surface, were subsequently 

 embedded in the first deposits of a succeeding sea, argue convincingly 

 for land conditions and subaerial decay of the limestone surface. This 

 and similar phenomena have been observed frequently. In all of the 

 four instances to be cited the succeeding formation is a soft shale, three 

 of them black shale, the fourth a green shale. The reason for their 

 selection will be mentioned presently. 



In the first, seen near St. Joe, Arkansas, the Chattanooga shale rests 

 on an uneven surface of Polk Bayou (early Eichmond) limestone. The 

 weathered out fossils of the latter were preserved as phosphatized casts 

 in a thin, imperfectly stratified clayey layer. This was followed by 

 normal fissile Chattanooga shale. 



The second example is shown in Calhoun County, Illinois, east of 

 Batchtown, where the eroded surface of a highly fossiliferous Devonian 

 limestone is succeeded by a greenish shale of Kinderhook age. 



The third case was observed in excellent exposures, 4 to 8 miles west of 

 Tahlequah, Oklahoma. The emergence seems to have been of relatively 

 brief duration and the area affected probably not very extensive. It 

 occurred after the deposition of a thin bed of Tennessean limestone (only 

 locally present in northwestern Arkansas and in northeastern Oklahoma), 

 and before the first shale of the Fayetteville group was laid down. These 

 two beds constitute the first and second members of the Fayetteville, as 

 described by Taff in the Tahlequah Folio of the II. S. Geological Atlas. 

 AVhatever the duration of the emergence in this case, surface decompo- 

 sition of the limestone continued long enough to form irregular undula- 

 tions and corrode its fossil contents. 



The fourth example was observed on Pilot Knob, 2 miles east of St. 

 Joe, Arkansas. Here the "coal bearing" shale, the third member or 

 formation of the Morrow group, rests directly on the eroded surface of 

 the Pitkin limestone. On removal of previously undisturbed shale, fossils 

 of various kinds were found imbedded in the limestone, but weathered ofl^ 

 flush with its worn surface so as to show in section. Other specimens, 

 proving more resistant, showed in relief. Both conditions are exactly 

 duplicated in present-day weathering of Pitkin limestone. 



These examples were selected for two reasons : first, because the over- 

 Iting shale is easily removed and thus permits examination of fresh sur- 

 faces; second, because the shale forms an impervious cover and thus 



