PROGRESS IN THE STUDY OF ANCIENT SEDIMENTS. 9 



sufficiently great to allow them to be included in one group. 

 At least two horizons of Olenellus are known elsewhere as 

 proved by Schmidt in the year 1888, and considering that 

 the Olenellus group is comparable in importance with the 

 Paradoxides group, which is divisible into several zones 

 characterised by different species of Paradoxides, as estab- 

 lished by the researches of Hicks, Brogger, and Tullberg, 

 and also that so many species of Olenellus are now known, 

 we may well suppose that the Olenellus beds like those 

 containing the Paradoxides fauna may be separated into 

 several zones characterised by different species of Olenellus. 

 It has not yet been made clear that the Neobolus beds are 

 not the equivalents of some of these, for the fossils associated 

 with Olenellus have not yet been described from all the 

 areas where Olenellus is found. 



Should the Neobolus beds be definitely proved to be 

 lower than the Olenellus beds, the question as to where the 

 base of the Cambrian system should be drawn, which is 

 bound to confront us sooner or later, becomes a matter of 

 pressing importance, for it will not do to absorb group after 

 group into the Cambrian rocks as they are found to be 

 fossiliferous. 



The Cambrian system as defined by Sedgwick is typically 

 developed in North Wales, where its base is formed by the 

 Harlech and Llanberis beds. In these North Welsh beds 

 the Paradoxides fauna is known to occur, but the existence 

 of strata of the age of the Olenellus beds is only inferred, 

 no definite fossils of Olenellus age having yet been detected 

 therein. Still, the Olenellus beds have so constantly been 

 referred to the Cambrian system that it would introduce 

 additional confusion to relegate them to an earlier system, 

 should it be ultimately proved (which is unlikely) that none 

 of the beds of Sedgwick's typical area appertain to the beds 

 of Olenellus age. Also considering that the Olenellus, Para- 

 doxides, and Olenus faunas are of approximately equal im- 

 portance, the beds containing them make a convenient 

 system as the term is understood amongst geologists. But 

 the inclusion of beds of earlier age than the Olenellus beds 

 (such beds being probably not represented in North Wales) 



