368 On a Ternary Geological Classification. [J^Jj 



Ireland, and as throwing light npon the mutual relations of the 

 ujDpor and lower members of that group. 



Jurassic Series. — I venture to assert that no very valid objec- 

 tion ctm be taken to the arrangement of the three Jurassic groups 

 here adopted, and which are given in the table of geological forma- 

 tions. If we examine the elaborate tables of the distribution of 

 species which Professor Eamsay has, with the aid of Mr, Etheridge, 

 laid before the Geological Society, we shall observe that there are 

 species common to the groups as here arranged, and that the theory 

 of " migration " accounts for the temporary change of groups of 

 animal remains. (See Plate.) 



The Liassic series seems to admit of a sub-group if we take the 

 marlstone (or Middle Lias) as the central calcareous member. 

 This, however, is a distinction to which it is scarcely entitled. The 

 u})per sedimentary member of the Upper Jurassic group is only 

 very partially represented by certain littoral and marine beds of 

 the Middle Purbecks. Their marine equivalents, and certain ante- 

 cedent strata now lost, are, however, the true representatives of this 

 stage. That these have been denuded away from off the upper 

 surface of the Portland limestone is, I think, made sufficiently clear 

 by the eroded sm-face which this rock generally presents. On the 

 Continent the same phenomena are presented, pointing to the 

 absence of certain strata through denudation. On this point M. 

 D'Orbigny says : — " Quant anx limites superieures rien nous manque 

 pour la separation nette et precise qui existe avec 1 etage ncocomien. 

 Cette separation, en effet, se montre sous tons les formes, par dis- 

 cordances et par des discordances de denudations et derosions." * 



Cretaceous Series. — The close of the Upper Jurassic period is 

 marked by a complete physical break, and, as above stated by 

 D'Orbigny, by denudations of the Jurassic beds. This is observable 

 all along the line of the chalk Downs through Wiltshire and Oxford- 

 shire, where wc find occasionally (as Dr. Fitton long since pointed 

 out) the Greensand resting on different members of the Jurassic 

 series, from the Portland Oolite down to the Coral Rag inclusive. 

 As regards the accompanying change in the paLieontological features, 

 Professor Ramsay remarks : — " The break in the succession of 

 species is as great as in any part of the Palaeozoic series, and is 

 shown by the total change of species which marks the introduction 

 of the marine cretaceous formations." t 



The Lower Greensand, Gault, and Upper Greensand, though 

 showing in places evidence of breaks in the succession^ form on 

 the whole a well-defuied lower sedimentary member of the Cretaceous 



* ' raU'onlolngie,' p. 5(J3. 



t Anniversary AddrcHs to the GcologiVnl Socioty of TiOndon, 18C4. 

 X This break is shown to he large in the case ol' the Lower Greensand, for out 

 of 28 species 2.33 are peculiar and .51 (or IS per cent. ) pass upwards. — Ramsay, ihid. 



