﻿Vol. 6 1.] ANNIVERSARY ADDRESS OE THE PRESIDENT. lxxxi 



For the divisions founded on faunistic change, the 

 name of a dominant organism or group of organisms should be 

 applied, as zone of Micr aster (sp.), zone of JStephanoceras Humplire- 

 sianum, zone of Monograptus gregarius. As the term zone has 

 been so generally used by stratigraphical geologists in the sense of 

 fossil-zone, it would be well to use it only in this way, it 

 being distinctly understood that the ^term is applied to a belt of 

 strata characterized by some organism or organisms, without any 

 regard to the thickness of that belt. 



It has been customary to apply local names to the lithological 

 belts, and this custom is in every way desirable. It is true that it 

 necessitates the employment of new terms when a fresh area is 

 worked out in detail, but the trouble thus caused is slight ; whereas, 

 if the name given in one locality be applied to another, endless 

 confusion may arise, should it be proved that the beds to which the 

 name is afterwards assigned are really of different age from those 

 to which it was first given. 



In illustration of these remarks, I wish to direct your attention 

 at some length to the strata of the Ordovician system, for 

 the classification of those rocks is still in an unsatisfactory state. 



The time-divisions which I would, at present, propose to adopt for 

 the rocks of this system are as follows : — 



[ Ashgillian. 



~ Caradocian. 



Ordovician < __ 



Llandeihan. 



I Skiddavian. 



I say at present, for should modification] be necessary in the 

 future, it can be readily adopted. 



In proceeding to discuss the significance of these terms and the 

 reason for their adoption, it will be convenient to consider them 

 seriatim, beginning with the oldest — namely, the Skiddavian rocks. 



1. The Skiddavian Series. — In Sedgwick's final revision 

 of the classification of the great mass of Lower Palaeozoic rocks, 

 which he studied with such success, he applied the term ' Arenig 

 or Skiddaw Group ' to the rocks which we are now considering. 1 I 

 suggest the use of the term derived from the Cumbrian rather 

 •than from the Cambrian hill, because the age of the rocks of 

 Arenig is still under discussion, while the Skiddaw Slates have 

 yielded a rich fauna which enables us to refer the great bulk of the 



1 ' A Catalogue of the Collection of Cambrian & Silurian Fossils contained 

 in the Geological Museum of the University of Cambridge ' 1873, p. 19. 



