234 AMADEUS W. GRABAU 
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 alternata, Plectambonites 
sericea, Dinorthis subquadrata, Plectorthis plicatella, Dalmanella 
testudinaria, Platystrophia bijorata; Protowarthia cancellata, Liospira 
micula, Clathrospira subconica, Trochonema umbilicatum, Camero- 
ceras proteijorme, Calymmene callice phala, Isoteles gigas, I. maximus, 
and Ceraurus pleurexanthemus. 
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 
