CORRELATION OF STRATA. 1 9 



seldom begun, continued and ended uniformly over wide 

 areas, we find that a formation based on such considerations 

 cannot always be definitely distinguished. 



Even in England the term Gault is generally understood to refer 

 to the clayey beds that occur between the Upper and Lower 

 Greensand, but the Gault may not be persistent as a clayey 

 stratum everywhere at this horizon : for instance, at the Blackdovvn 

 Hills in Devonshire it may be represented in part by sand. It 

 would, however, tend to produce confusion in our geological tables 

 were we to designate a certain part of the Blackdown Greensand as 

 "Gault." Where the Kimeridge and Oxford clays come together 

 without the intervention of the Corallian series, we might identify 

 portions of the clay as probably synchronous with the absent lime- 

 stones and sands, but it would obviously be absurd to term portions 

 of the clay "Coral Rag." Our minor geological terms must be 

 applied essentially to lithological divisions, for these only can be 

 represented on geological maps. 



At the present day much attention is paid to minute palseonto- 

 logical divisions, and to assemblages of fossils or zcmes, to which a 

 name is given according to the occurrence of some prominent 

 species. These zones are most useful in indicating the succession 

 of life, but they are, as a rule, irregular in their limits. The term 

 Horizon is used to designate the particular level or position at 

 which a certain stratum or fossil may occur in a formation. 



In classifying and correlating different deposits there are 

 many circumstances to be taken into consideration, and it 

 will be seen that, in comparing the formations of countries far 

 apart, it is often impossible to prove contemporaneity, and 

 that the term ' homotaxial,' used by Prof. Huxley, is more 

 convenient to express similar relative position without neces- 

 sarily meaning identical age.^ 



Although, in the following pages, descriptions of each for- 

 mation will be given, it must be borne in mind that such 

 descriptions refer only to the main or characteristic litho- 

 logical features. Hand-specimens might be collected which 

 would be characteristic of each formation, and yet specimens 

 might be brought together of Silurian, Carboniferous, and 

 Lias Limestones, which would seem identical in character ; 

 or of Lias Clay, Oxford Clay, and Kimeridge Clay, which 

 could not be distinguished ; or of Old Red Sandstone, Coal- 

 measure Sandstone, and New Red Sandstone of very similar 

 composition. 



The life-history of each formation tells of a plan which 



^ Address toGeol. Soc. 1S62 ; see also H. G. Seeley, Ann. Nat. Hist. Dec. 1S67. 



