736 C. 0. Dunbar — Stratigraphy and Correlation 



nessee that disconformities are the rule and unconformi- 

 ties the exception, even where the break in sequence is 

 known to be a long one. 



Lower Devonian Series 



Linden or Helderbergian Group 



Rockhouse shale. — Heretofore the Ross limestone has 

 been regarded as the lowest member of the Linden, and 

 its base as the beginning of the Devonian sequence in 

 Tennessee. But in southern Hardin County a still lower 

 undescribed formation of f ossilif erous shale comes in like 

 a wedge between the Ross and the Decatur (Silurian) 

 limestone. This formation thickens to the southward, 

 where it goes permanently below drainage. A maximum 

 thickness of 26 feet may be seen at Rockhouse, a hunters' 

 clubhouse on Horse Creek, 5 miles northwest of Lowry- 

 ville, and because of this good exposure the formation 

 will be named the Rockhouse shale. It is much thinner 

 where it forms a glade near the sulphur spring on Horse 

 Creek, 5 miles southeast of Savannah, and it does not 

 appear in the sections farther north. It is a glade-form- 

 ing, calcareous shale of greenish gray color, interbedded 

 with occasional thin bands of light gray crystalline 

 limestone which toward the base of the shale become 

 thicker and closer set, so that the formation appears to 

 grade into the underlying Decatur limestone. 



The shale is replete with fossils, among which the 

 bulbous crinoid root Camarocrinus is most conspicuous. 

 The fauna consists of thirty-five species, of which one 

 third are new. The assemblage is an extremely interest- 

 ing one, with a mingling of holdovers from the Silurian, 

 along with heralders of early Devonian time. It indi- 

 cates for the formation a position very early in the Devo- 

 nian and near the Siluro-Devonian boundary line. The 

 interesting biota includes Edriocrinus poeilliformis, E. 

 adnascens n. sp., Camarocrinus, Scyphocrinus sp., Pleu- 

 rodictyum trifoliatum n. sp. (ancestral to P. lenticular e 

 but mature at the three-celled stage), Dalmanella macra 

 n. sp., D. rockhousensis n. sp., Rhipidomella oblata, R. 

 preoblaia, R. saffordi, Bilobites (small form like bilobus), 

 Leptcenisca adnascens, Dictyonella subgibbosa n. sp., 

 Eatonia fissicosta n. sp., Delthyris cyrtinoides n. sp., 

 (very near perlamellosa), Nucleospira concentrica, Mer- 



