NASHVILLE SECTION 577 



conditions, but south of Franklin I saw a large surface of one of the pure 

 layers which exhibited deep shrinkage cracks, forming a pattern of large 

 polygons. The more probable interpretation is, therefore, that these beds 

 are the deposits of lagoons, which were so shallow that at times they were 

 entirely deprived of water and exposed to desiccation. Taking these 

 strata and the Capitol limestone below them, they together suggest con- 

 ditions which obtain at the present time about reefs in the tropical seas. 



JO. Massive gray limestone with nodules and extensive lenticular layers of 

 chert — 26 feet. This is the Ward limestone of Jones. 



11. Thinly bedded gray to buff limestone, which rests unconformably on the 



layers below and is itself truncated by the strata above — 4 feet. 



12. Pure buff limestone, which increases from a layer of nodules in the west- 



ern part of its exposure to two beds, each about 1 foot thick, at the 

 eastern — 0-2 feet. 



Zones 11 and 12 appear to represent beds formed just at the margin 

 of a lagoon, and that they cut across the strata beneath has only a limited 

 significance till it can be determined how much has been cut out. 



Safford (Geology of Tennessee, page 277) called attention to these two 

 zones, which have a variable thickness in the rather numerous exposures 

 about Nashville. Number 11 he called the Cyrtodonta bed and stated 

 that ill the bluff at the wire bridge it was a single bed, 10 or 11 feet thick, 

 made up of the shells of Cyrtodonta saffordi, Bellerophon lindsleyi, and 

 B. troosti. 



"The bed is seen at the engine-house of the water-works, where it is 6 feet 

 thick. In tracing it beyond the engine-house it very soon runs out and is 

 replaced by a compact dove-colored limestone." 



Paul M. Jones, in a little-known paper, 9 which 1 have seen through the 

 courtesy of Mr. W. A. Nelson, has described in some detail the character- 

 istics and distribution of the Cyrtodonta bed and the "Upper" or "False 

 Dove." The former is stated to be a very local development, occupying 

 only a small area near Nashville. It varies in thickness from to 11 

 feet, and is usually a single bed, made up of an aggregation of molluscan 

 shells. The "False Dove" is from 2 to 5 feet thick, and contains ostra- 

 cods, gastropods, and cephalopods, with an occasional Tetradium. Al- 

 though it has, on the weathered surface, much the same appearance- as 

 the "Bigby Dove," it is really a much less pure and less compact lime- 

 stone. 



13. Massive limestone with numerous bryozoa, succeeded by rather shaly 



strata. Base of the Catheys — 17 feet. 



9 The geology of Nashville and immediate vicinity. Nashville, 1802. 



