418 R. M. Field — Middle Ordovician of Central 



adopted by the United States Geological Survey without 

 a clear or sufficient definition, the description of the for- 

 mation not being that of the type locality. The propri- 

 ety of the use of the term Center Hall, strictly from the 

 point of view of the map-maker, is perhaps open to argu- 

 ment. It must be remembered, however, that the ulti- 

 mate goal of the stratigrapher is not a more or less neatly 

 drawn columnar section but a comprehensive view of the 

 paleogeography of the area in which he is at work, and 

 failure to recognize certain zones in the Bellefonte sec- 

 tion would lead to an incorrect picture of the upper 

 Stones River sea. 



Summary of the Stones River Reef. — The stratigraph- 

 ical relationship of the Carlim and Valentine formations 

 strongly supports the hypothesis that the former was a 

 "bedded reef" or shallow-water platform upon which 

 flourished, from time to time, dense mats of colonial 

 organisms, while the latter represents the (deposits of a 

 broad lagoon, or shallow-water shelf, protected from the 

 action of the open sea. The columnar sections of fig. 2 

 indicate that the Valentine thins out in a southeasterly 

 direction and that its place is taken by an increased 

 thickness of the Carlim. Figure 3 is a diagrammatic 

 cross-section in a southeasterly direction from Bellefonte 

 through Pleasant Gap and Center Hill to Reedsville. 

 Although the Lemont, Center Hill, and Coburn sections 

 lie respectively to the southeast and northwest of this 

 line, they have been projected upon it for the sake of 

 producing all the evidence upon one diagram. 



The basal beds of the Carlim were laid down upon the 

 Loysburg platform in very shallow water, as is indicated 

 by the abundance of mud-cracks, ripple-marks, and edge- 

 wise giomerates previously mentioned. During the 

 early stages of the accumulation of the formation, con- 

 ditions were suitable for the growth of attached colonial 

 as well as other benthonic types which at times formed 

 mats or beds of appreciable thickness and considerable 

 area. Tetraxlium syringoporoides, T. fibratum, T. cellu- 

 losum, Girvanella, and numerous species of Bryozoa 

 were important reef -building types. 



In the closing stages of the Carlim, Columnaria became 

 relatively more abundant toward the continental side. 

 Toward the west, or open sea, conditions were apparently 

 not suitable for the growth of the reef, and the pure 



