742 C. 0. Dunbar — Stratigraphy and Correlation 



would be a most unfortunate one to apply to the forma- 

 tion, since the limestone at Pyburns is an isolated occur- 

 rence about 40 miles south of the nearest good section of 

 the formation under discussion. Moreover, it is neither 

 faunally nor lithologically typical or representative of 

 the latter. It is the conclusion of the present study, how- 

 ever, that the Birds ong formation is distinct from and 

 younger than the Pyburn limestone, and it is therefore 

 given this new name because of its typical development 

 along the valley of Birds ong Creek. 



At the base of the formation is 8 to 10 feet of rather 

 thick-bedded, coarsely crystalline, gray limestone, fol- 

 lowed by a transition zone of thinner bedded crystalline 

 limestone, interbedded with bluish calcareous shale, pass- 

 ing into bluish shaly limestone or limy shale which forms 

 the upper half or two thirds of its thickness. It weathers 

 into bluish clay and a rubble of small lumps of limestone, 

 and frequently forms barren hillsides or "glades." 



The formation attains a thickness of about 45 feet 

 along Birdsong Creek, and continues with but little 

 change, either lithologically or faunally, through north- 

 ern Decatur County and Benton County, though it is 

 generally below drainage in central and northern Benton 

 County. About 5 miles above the mouth of Big Sandy 

 River, where it comes to the surface again, it reveals an 

 additional 20 feet of higher shaly layers not to be seen 

 further south. To the south and east of Perryville it 

 has been beveled off by interformational erosion, but a 

 small outlier still persists in the vicinity of Saltillo. A 

 thickness of only 8 feet of these strata is exposed at the 

 boat landing at this locality. The association here of 

 Anastrophia verneuili, Dalmanella eminens, and Eospiri- 

 fer macropleura (very common) leaves no doubt of their 

 reference to the Birdsong formation, and they apparently 

 do not represent even the lowest layers, so that the for- 

 mation appears to overlap to the south. 



In all of the sections west of the Tennessee River this 

 formation rests with a disconformable contact upon the 

 Decatur limestone of the Silurian, the boundary line 

 between them being usually difficult to locate. Farther 

 northeastward, at Beardstown on Buffalo River and near 

 Cumberland City in the Wells Creek basin, it overlaps 

 younger Silurian strata. 



Excepting the limestone at its base', the Birdsong for- 



