PHYSICAL AND FAUNAL EVOLUTION 69 



of great importance. It certainly does not represent Utica shale. 

 There is, however, a marked and widespread disconformity between 

 the Lower and Upper Trentonian, the late Richmond resting on 

 Trenton or even earlier beds. This is observed throughout the Rocky 

 Mountain area, the upper Mississippi region, and to a less extent in 

 other sections. It signifies a retreat of the sea, probably at the end 

 of Trenton time, and a return during late Richmond time. 



THE TRENTON-CINCINNATI FAUNAS 



While on the whole the faunas of the Trenton limestone and of 

 each one of the three divisions of the Cincinnati group are sufficiently 

 distinct, so that it is not difficult to recognize the exact horizon of 

 each by a careful analysis of the fauna, there is, nevertheless, a unity 

 in these faunas, which shows their unmistakable relationship to one 

 another and their distinctness from the preceding faunas. It is 

 this broad similarity of faunas, together with the distinctness from the 

 preceding faunas, the intimate relation of the limestone to the Utica 

 shale which it replaces, and the moderate thickness of the formation 

 in its best development, as compared with that of the Chazy and 

 Beekmantown, that has led me to place the Trenton limestone in the 

 Upper Ordovicic. In England, the Upper Ordovicic or Caradocian 

 (Bala) is characterized by the same faunal elements which here 

 appear for the first time. The more common species characterizing 

 the Upper Ordovicic from the Trenton up, and occurring in most 

 if not all of its beds, include Rafinesquina aUernata, Plcciamhonites 

 sericea, Dinorthis subquadrala, Plectorthis plicatella, Dalmanella 

 iestudinaria, Platystrophia hijorata; Protowarthia cancellala, Liospira 

 tnicula, Clathrospira subconica, Trochonema iimbilicatum, Camero- 

 ceras proteijorme, Calymmene c alike phala, Isoldes gigas, I. niaximus, 

 and Ceraurus pleurexanthennis. 



Some of these species begin in the Black River or even in the 

 Upper Stones River, but they are most characteristic of the higher 

 horizons. 



THE CONTINENTAL PHASE OF UPPER ORDOVICIC TIME 



The later epochs of Upper Ordovicic time were characterized 

 by continental or non-marine sedimentation in the Appalachian 

 region. The earliest of these is the conglomeratic and quartz- sand 



