542 ME. E. E. L. DIXON AND DK. A. VATTGHAN ON [NOV. I9II, 



VI. NOTES ON THE DELIMITATION OF THE UPPER FKOM THE LoWEE 

 AvONIAN IN THE SoUTH-WesTEKN PeOVINCE. 



(1) Stratigraphical Considerations [E. E. L. D.]. 



The earth-movements discussed in the previous section (pp. 536- 

 37), and the faunal, lithological and stratigraphical changes which 

 are connected with them, afford a basis for a delimitation of the 

 divisions of the Avonian in the South- Western Province. In 

 applying this principle, it is advisable to include a lagoon-phase in 

 the base of a division rather than to associate it with the under- 

 lying rocks, because such phases, in general, mark the commence- 

 ment of cycles of earth-movement. 



For these reasons the base of the Modiola phase at the base 

 of (Ca + SJ, that is, the divisional line between C x and C 2 , is the 

 appropriate horizon at which to separate, as is necessary, an Upper 

 from a Lower Avonian. 1 It is the more suitable because it is sharply 

 denned lithologically and, in places, is emphasized by unconformity 

 with the underlying rocks. And this unconformity, there is reason 

 to believe, is the commencement of the transgression that has brought 

 the Upper Avonian to rest upon far older rocks in the northern part 

 of the South Wales area. 



In those places, however, which represent the deeper parts of 

 the Avonian sea, not only does the unconformity disappear, but 

 also, as the distance from the Avonian coast increases, the Modiola 

 phase itself is lost, and ultimately the base of C 2 may become un- 

 traceable by lithological means. Faunal methods, however, are 

 available, such as is afforded by the incoming of Cyathophyllum <j> 

 Vaughan, though, as will be seen from the following note by 

 Dr. Yaughan, the extreme base of C 2 does not everywhere corre- 

 spond to the maximum of the change of fauna in passing from the 

 Lower to the Upper Avonian. 



(2) The Faunal Relations of C 2 to the Beds below 



and above [A. V.]. 



Wherever, as at Burrington, etc., C x is composed of limestones of 

 standard type, the C x fauna is a continuation of the y facies, that 

 is, an extension and amplification of the Lower Avonian fauna, 

 and, locally, this fauna is persistent into the base of C 2 (as here 

 defined on lithological grounds). 



On the other hand, the main C 2 fauna exhibits the earliest 

 establishment of genera and gentes which are especially character- 

 istic of Upper Avonian time. For example : — 



Corals: — 



The gens of Cyathophyttum aff. murchisoni. 



The genera Gampophyllum , Diphypkyllum, and Clisiopkyllum. 



i The terms Upper and Lower Avonian are used provisionally, pending 

 a general discussion of the Avonian of this and other areas, in some of which 

 other terms, such as Visean and Tournaisian, are already in use. 



