206 SECONDARY. 



Although on the whole the Secondary strata lie uncon- 

 formably upon the denuded Palaeozoic rocks, yet, as Lyell 

 remarked, " nowhere have geologists found more difficulty in 

 drawing a line of separation than between the Secondary and 

 Primary series," for there is no great palseontological break 

 between them. Thus some geologists include the Permian 

 rocks in the Palaeozoic era, while others associate them with 

 the Triassic rocks on account of the similarity in their cha- 

 racters and method of formation. This latter grouping is 

 here adopted. 



The formations from the New Red Sandstone to the 

 Purbeck and Wealden Series are generally speaking con- 

 formable. In places these strata were upheaved and much 

 denuded before the overlying Cretaceous rocks were laid 

 down : for the Chalk and Upper Greensand overstep the out- 

 cropping edges of the older Secondary strata, and transgress 

 from the Purbeck and Wealden Beds of Dorset to the New 

 Red Marls and Sandstones of Devonshire. In Yorkshire, 

 also, there is a similar overstep of the Upper Cretaceous rocks. 



The following are the Secondary systems : — 



Cretaceous, | t 



( Lower. 



T • i Oolitic Series. ) 



urassic. \ -r ■ • c • t\t • i 



•' ( Liassic series. > JMercian.^ 



New Red Sandstone or Poikilitic. ) 



The Secondary rocks are characterized by Marsupial Mam- 

 malia ; by numerous remains of Reptilia, such as the IcJithyo- 

 smirns and Plesiosannis, the Ccteosaiims and the Megalosmn'iis ; 

 by Mollusca, of the genera Annnonites, Bcleiunites, Pleiiroto- 

 inaria, Pholadoviya, Lima, and Trigonia ; by the Brachiopoda 

 RJiyncJionella and Tercbvatida ; by Corals of the genera 

 Montlivaltia, Thecosinilia, Isastrcua, and Thaniiiastrcca ; and 

 by Cycadeiform plants. 



^ Term suggested by Prof. T. McK. Hughes, Proc. Cambridge Phil. Soc. 

 iii. 247. 



