T. E. Savage — Ordovician and /Silurian Formations. 513 



been described by Ulrich and Hayes,* for which the name 

 Fernvale was proposed, from the town of Fernvale, in William- 

 son county, Tennessee. From the similarity of the fossils in 

 the two areas this limestone overlying the Kimmswick forma- 

 tion in the vicinity of Thebes is considered the equivalent of 

 the Fernvale beds in Tennessee, and the name of the Tennessee 

 locality has been used to designate this basal Richmond forma- 

 tion in Alexander comity. 



This horizon is exposed at only two points, at each of which 

 the area of outcrop is very limited in extent. A thin zone 

 may be seen on the top of the Kimmswick blocks in the bed 

 of the river, one-fourth mile north of Thebes. A thickness of 

 three and one-half feet of this limestone occurs immediately 

 underlying the Thebes sandstonef in the south part of the 

 town of Thebes. 



Among the fossils found in this limestone are bulbous cri- 

 noid segments, Dinorthis subquadrata, Hebertella inscidpta, 

 H. occidentalism JPlaty atrophia acutilirata, Plectorthis whit- 

 Jieldi, Rafinesquina alternata, Rhynchotrema capasc, Stropho- 

 mena fluctuosa and S. planumbona. 



Correlation. — The continuity of the Richmond sea in Illi- 

 nois and Iowa was apparently broken by a number of low 

 land barriers extending in a general northeast-southwest direc- 

 tion. The sediments of this age in Iowa and northwest Illi- 

 nois have been called the Maquoketa beds. The sea in which 

 they were laid down was not broadly connected with that in 

 which the Richmond beds in the southern and eastern parts of 

 the state were deposited. For this reason exact correlation of 

 horizons in the two areas is as yet difficult. 



In the Maquoketa beds of Fayette county 4 Iowa, Rhyncho- 

 trema capax occurs at three successive horizons. It is found 

 first in the lower Maquoketa division, in beds of alternating 

 shale and impure limestone, a short distance above the zone 

 of Nileus vigilans (JSTo. 5 of the general section on page 485 

 of the Fayette County report). The second appearance is in 

 the limestone or dolomite which constitutes the middle divi- 

 sion of the Maquoketa beds, while the third occurrence is in the 

 alternating shale and limestone layers near the top of the 

 upper Maquoketa beds. Among the fossils associated with 

 Rhynchotrema capax in the lowest horizon are Dinorthis sub- 



* Ulrich and Hayes ; The Columbia Tennessee Folio, No. 95, U. S. G. S., 

 1903. 



f Note : The position of this horizon is immediately below 2a of the gene- 

 ral section given in the preliminary statement. (This Journal, vol. xxv, p. 

 443, 1908.) It was not noted in that paper because its presence had not 

 been detected, nor had it previously been recognized in this portion of the 

 state. 



{Savage : Iowa Geol. Surv., vol. xv, pp. 484-486. 



