414 A. F. FOERSTE LIMESTONES OF TENNESSEE AND KENTUCKY 



neath, and under this, but best exposed a short distance southward, is 

 white limestone containing very little chert. The thickness of this white 

 limestone is at least 17 feet, but the base of the Clinton is not seen. The 

 presence of considerable chert in the upper part of the Clinton section, 

 accompanied by a comparative absence of chert in the lower part, is a 

 feature often observed in the southern part of the area under investiga- 

 tion. While not a constant feature, it nevertheless frequently serves to 

 give a clue as to the probable part of the Clinton section exposed. This 

 may be verified by exposing the rest of the section by digging. 



East of Palestine, on the east side of the creek (locality 31), the Black 

 shale rests on 18 feet of Clinton, the upper 6 feet with chert nodules, the 

 lower 12 feet more heavy bedded. East of the mouth of Little Swan 

 creek (locality 32) the Black shale rests on 12 feet of heavy bedded 

 Clinton. A third of a mile up stream (locality 33), 17 feet of Clinton 

 are exposed beneath the Black shale, the lower beds being heavy 

 bedded. Still farther up stream, about three-quarters of a mile below 

 the crossing of the Natchez trace, and an equal distance beyond the 

 last localit}^ the Clinton beneath the Black shale is only 4 feet thick. 

 It forms a good quarry rock, and where freshly exposed shows a light 

 blue color and a dense grain. Above Gordensburg the Silurian is ab- 

 sent, and the Black shale rests on the Ordovician. 



CONTRAST OF VARIATIONS IN THICKNESS OF SILURIAN SUBDIVISIONS 



Where the various subdivisions of the Silurian are overlaid by the 

 Devonian the.y usually vary considerably in thickness. This variation 

 in thickness generally consists in a rapid diminution toward the margin 

 of outcrop along the flank of the anticline. ' Considering the gradual 

 variations in thickness of these subdivisions where overlaid by higher 

 subdivisions of the Silurian, their rapid diminution in thickness where 

 no longer overlaid by higher Silurian formations is very striking. Since 

 all of the subdivisions of the Silurian are affected in this manner, a con- 

 siderable section often disappears within a very short distance (figures 

 1,3,4,5). 



At Bledsoe (locality 3) the Silurian section is 151 feet thick, but no 

 trace of it is seen at the Gap of the ridge, 9 miles eastward. This is a 

 decrease in thickness of about 17 feet per mile. Between the S. R. 

 Wood locality (locality 2) and the Gap, however, the rate of decrease is 

 about 30 feet })ermile, while the indications are that between the Halum 

 P. Weeks locality (locality 1) and the Gap a Silurian section of 31 feet 

 disappears within considerably less than a mile; and yet there is every 

 reason to believe that the full section of Osgood clay shale is present 

 at the Halum P. Weeks localit}^ and that this clay shale, where con- 



