Outlines of Geology. 85 



chalk forms a very conspicuous feature ; but there are, lying upon 

 it, a variety of clays and sands, and beds of gravel, which will 

 claim our earliest notice. Beneath the chalk, another and a dis- 

 tinct series of sands and clays will be found to prevail, which are 

 incumbent upon calcareous freestone and lias : to these succeed 

 marls and sandstones deeply tinctured by red oxide of iron, and 

 often containing detached masses, as it were, of rock salt and 

 alabaster. These may be considered as constituting one compre- 

 hensive and important subdivision of the strata. The next in- 

 cludes the coal deposits, and the sandstone and limestone with 

 which they are more immediately associated, or upon which they 

 are incumbent. The third subdivision is characterized by the 

 prevalence of slaty or schistose rocks ; and the fourth is confined 

 to granitic aggregates, of which there are many sub-species. 



We thus find the coal strata interposed between two great fa- 

 milies of rocks, which, with Messrs. Conybeare and Phillips, we 

 may call the supermecUal and submedial orders ; coal and its asso- 

 ciates are the medial order, and granites on the one hand, and 

 alluvial and diluvial matters on the other, form the inferior and 

 supericrr series. Using these terms, we lose sight of all theore- 

 tical distinctions — of primary, transition, and secondary for- 

 mations. 



The medial order of rocks, and all above them, are charac- 

 terized by containing remains of vegetable and animal tribes ; 

 and these are sometimes in great profusion ; but they are far 

 from being indiscriminately scattered through the strata; on the 

 contrary, " they are disposed as it were in families, each forma- 

 tion containing an association of species, peculiar in many in- 

 stances to itself, widely differing from those of other formations, 

 and accompanying it throughout its whole course ; so that at two 

 distinct points on the line of the same formation, we are sure of 

 meeting the same general assemblage of fossil remains." 



In the carboniferous limestone, for instance, forming, as we have 

 said, a member of the medial series, we always find the same 

 corals, encrinites, terebratulae, <5r., from whatever part of the 

 world our specimen comes. In the chalk, too, there is an asso* 



