CENTRAL TENNESSEE SECTION 575 



The Hermitage rests upon the Carters, which is generally, although 

 perhaps not everywhere, of Leray-Black River age. 



Having enumerated these formations, beginning with the youngest, 

 they may be best considered in reverse order. 



The Hermitage is chiefly remarkable for the extraordinary abundance 

 of specimens of Dalmanella fertilis Bassler, a shell which in many places 

 occupies the layers to the exclusion of all other fossils. It is, in fact, 

 the guide fossil, and it is the usual practice to place the top of the Hermi- 

 tage at the horizon where this brachiopod ceases to be abundant. Curi- 

 ously, even it is not common in Rutherford County, where the lower 50 to 

 70 feet of the Trenton are nearly unfossiliferous. 6 



The Bigby is an exceedingly interesting formation, with a highly di- 

 versified development in various parts of the central basin. Its charac- 

 teristics in the typical region have been mentioned above. In Davidson, 

 Franklin, and more eastern counties it is chiefly remarkable for the de- 

 velopment it shows of the so-called "Bigby Dove" or "Upper Dove" strata. 

 These consist of thickly bedded, pure, fine grained buff limestone which 

 greatly resembles the Lowville or "Birdseye" and which contains an 

 abundance of Tetradium and Ostracoda along with some other fossils, 

 chiefly mollusca. At Nashville these beds are not thick, but farther east- 

 ward they become the Cannon formation of Ulrich 7 and reach a con- 

 siderable thickness. About this development almost nothing has, how- 

 ever, been published. According to Ulrich, 8 the thickness of the Bigby 

 between Hartsville and Carthage (in the northeastern part of the basin) 

 is between 120 and 150 feet and .more than half of it is "dove." The 

 fauna, moreover, while profuse, is mostly molluscan, bryozoa and brachio- 

 pods being almost absent. 



The discussion can, perhaps, be made more concrete by reviewing the 

 well known section at Nashville. The following measurements were 

 made by the writer in 1921, and for convenience in reading are given 

 in ascending order. 



The Section at Nashville 



The lowest layers of the Trenton are most conveniently studied in the 

 cuttings along the Nashville, Chattanooga, and Saint Louis Railroad in 

 the southeastern part of the city. The Hermitage rests here upon the 

 rather thinly bedded upper layers of the Carters. 



6 Galloway : Tenn. Geol. Survey, Bull. 22, 1919, p. 51. 



7 Bull. Geol. Soc. Am., vol. 22, 1911, p. 417. 



8 Columbia folio, 1903, p. 2. 



