72 STRATIGRAPHICAL GEOLOGY 



the Harlech Series was found to be divisible into two groups (the 

 Caerfai and the Solva Beds). It has, moreover, been found that the 

 whole succession is divisible into series which are characterised by 

 special genera of trilobites, and that in many places a further sub- 

 division can be made into zones based on the limitation of certain 

 fossils (see p. 22). Thus the old lithological grouping has been 

 modified to agree with the pal aeon tological evidence, and the best 

 modern arrangement of the rock-groups that compose the Cambrian 

 system in England is as follows : 



Upper {; 



Tremadoc Beds = the Bryograptus fauna. 



Lingula Flags (in three stages) = the Olenus fauna. 



Middle { S^eds 136 ^ } = the Pam doxides fauna. 



Lower = Caerfai Beds = the Olenellus fauna. 



B. LIFE OF THE PERIOD 



Great interest attaches to the fossils which have been found in 

 the Cambrian rocks, because, with the exception of the few organic 

 remains mentioned on p. 61, they include the remains of the oldest 

 animals hitherto discovered. At the same time the number of 

 different classes of creatures which exist in the oldest Cambrian 

 fauna make it certain that ancestors of these creatures must have 

 existed in pre-Cambrian times, and there can be little doubt that 

 some of them will sooner or later be discovered in the higher 

 pre-Cambrian rocks. 



The fossils which have been found in the Lower and Middle 

 Cambrian of Wales and Shropshire are referable to the following 

 groups Porifera, Cystidea, Annelida, Brachiopoda, Gastropoda, 

 and Crustacea ; while representatives of Actinozoa, Crinoidea, 

 Graptolitoida, and Lamellibranchia have also been obtained from 

 rocks of the same age in North America. The very existence of so 

 many different kinds of animals at this early period is a remarkable 

 fact, and is a proof of the great gap which exists between the 

 Cambrian and the pre-Cambrian rocks of Western Europe. 



But although referred to many different classes, examination of 

 the fossils themselves reveals the fact that most of them are repre- 

 sented by lowly and more or less generalised forms that is, forms 

 which combine in themselves characters that are now only found 

 in two or more distinct families or orders of beings. Thus Archceo- 

 cyathus and its allies have been classed by some with the Sponges 

 and by others with Corals, and they are now regarded as a primitive 

 family intermediate between Porifera and Actinozoa, and possibly 

 representing the stock from which both these classes were derived. 



