C. Lapworth — Classification of the Loiver Palaeozoic Rocks. 11 



choose, if possible, the longest non -fossil iferous periods to divide 

 them, and these are almost certain to occur where there is the greatest 

 appearance of unconformability. 



Nevertheless, the same effect may be owing to a cause in its nature 

 diametrically opposite — the required palaeontological break being due 

 to a more than ordinary depression of the sea-bed, and the conse- 

 quent cessation of almost all deposition in that area — a circumstance 

 to my mind of equal importance from a classificatory point of view 

 with an unconformability itself. An extraordinary regional depres- 

 sion of this character seems to have been the actual cause of the appa- 

 rently sudden change in the facies of the Palaeozoic fauna at the 

 commencement of the Arenig period, both in Britain and Scandinavia, 

 and when fully worked out will, in all probability, enable us to lay 

 down a pakeontological line of demarcation far more strictly syn- 

 chronous throughout its geographical range than that which we shall 

 be compelled to adopt at the base of the Lower Llandovery. 



Nor is the venerable objection — that, owing to the established laws 

 of scientific nomenclature, a moral obligation is binding upon us to 

 adhere rigidly to the limits of each system as originally laid down 

 by its founder — worthy of a whit more respect. 



This is a claim whose absurdity verges upon the ridiculous when 

 it is advanced by the Murchisonian in support of his contention that 

 the Paradoxides and Olenus beds appertain to the Silurian, for they 

 actually antedate all the strata of Murchison's original Silurian 

 System. It is, therefore, only occasionally employed by him in a 

 restricted sense in defence of his retention of the strata of the Second 

 Fauna. 



It crops up continually, however, in the writings and arguments 

 of those belonging to the opposite party. It is urged again and 

 again with a wearisome iteration, as if this conservative rule in 

 geologic nomenclature were necessarily to over-ride every other 

 scientific canon whatsoever. But even if we grant that Sedgwick, 

 and not Murchison, first correctly defined and characterized the rock- 

 group which yields the Second Fauna, this rule is equally inoperative 

 in the face of our present recognition of the grand geological impor- 

 tance and distinctness of this Fauna. Of this fact Sedgwick was 

 originally wholly unaware ; nor does he ever appear to have 

 estimated it at its true value. To us, however, who have watched 

 the gradual elimination of the Primordial Fauna, the grand distinct- 

 ness of the Second Fauna is so glaringly apparent, that it is impossible 

 for us to conceive of the rock-group which it characterizes as a mere 

 subdivision of the Cambrian. 



It is all very well to plead for historic justice, and to demand, out of 

 respect to the memory of a genius, the adoption of the nomenclature 

 which the general geological world was, in a sense, deprived of the 

 opportunity of accepting during his lifetime. But time and geolo- 

 gical convenience will soon make short work of any scheme of 

 nomenclature, however historically just, if it be not in all its parts 

 the natural expression of the inter-relationships and mutual subor- 

 dination of the facts it is its special aim to associate and systematize. 



