74 SUBDIVISION OF STRATA. 



are enabled to frame very plausible conjectures concerning the condi- 

 tion of the waters under which the several strata were accumulated. 



General Terms. In the same way as a number of similar laminae 

 are sometimes united into one bed of stone, so several similar beds of 

 stone are sometimes associated into one rock, to which a specific name 

 is applied, as the Oolite, the Lias limestone, &c. 



Sometimes several of these rocks are grouped under the title for- 

 mation, as the Bath Oolite formation. Thus the Lias limestone beds, 

 the Lower Lias clay, Marlstone beds, and Upper Lias clay, are all in- 

 cluded in the Lias formation, which rests upon the New Eed Sandstone 

 formation, and is covered by the Bath Oolite formation. 



The International Geological Congress has recommended that the 

 largest series of Geological Deposits, such as Primary or Secondary, 

 should be termed a Group; the Group should be divided into 

 Systems, such as Cambrian System, Silurian System, &c.; the System 

 is to consist of Series ; the Series is made up of Stages ; and each 

 Stage may be resolved into Beds. These terms have corresponding 

 names to indicate divisions of time; thus 



Sedimentary Terms. Chronological Terms. 

 Group, Era, 



System, Period, 



Series, Epoch, 



Stage. Age. 



From these names the familiar English term Formation is omitted, 

 because it has been used on the Continent to indicate the mode of 

 accumulation of a deposit, instead of the deposit itself ; but it may 

 be long before English writers entirely give up this equivalent for 

 the term Series. 



Groups of British Strata. The whole series of British strata are 

 grouped, according to their relative antiquity, into three leading 

 divisions the Primary, or Palaeozoic; Secondary, or Mesozoic; and 

 Tertiary, or Cainozoic strata ; it being understood that such divisions 

 are chiefly adopted for convenience, as expressing with considerable 

 accuracy certain general analogies of origin, composition, and organic 

 contents, which prevail amongst the members of each division, but 

 yet are not to be considered as exclusively belonging to them. 



Two of these three divisions are again subdivided, upon exactly the 

 same principles, into systems of strata, which are marked by certain 

 recurrent rocks, striking analogies of composition, organic remains of 

 similar types, and positions derived from convulsions of the same 

 geological epoch. 



The systems are again usefully divided into formations or series ; 

 these into their several component stages or rocks; whose ultimate 

 analysis gives the strata, beds, and laminae of composition. The 

 superficial accumulations of gravel, sand, peat, &c., are classed under 

 the head of alluvial deposits. 



The Tertiary or Cainozoic Group of Strata are partly lacustrin 





