XIY. 



The term "age" is frequently used in the succeeding pages. 

 It is to be understood as capable of a general application in 

 speaking of divisions of the time scale, \vhether large or 

 small. The term age, is also used in designating the time 

 occupied in the formation of a particular stratum, or sub- 

 stage, thus : Encrinal age, Moscow age and Styliolina age, 

 or age of the Styliolina limestone. 



The unit of classification in the formation scale is the 

 stratum. Each stratum comprises a section of the forma- 

 tion, \vhich consists throughout its thickness of the same 

 rock material. Thus the Encrinal limestone is a stratum. 

 Similarl}^ the eight or nine feet of black uniform carbona- 

 ceous Genesee shales form one stratum. The stratum may 

 be subdivided into beds, of which, in a thick stratum, there 

 ma}' be many. The bed may again be divided into layers, of 

 which there may be several in one bed. A rock formation or 

 terrane may consist of a single stratum, or of a number of 

 strata, according to the magnitude of the division under 

 consideration, but all the related strata of that particular 

 division are included in the term formation. Thus the 

 Devonian formation includes all the strata of the Devonian 

 division, and the Genesee formation includes all the strata of 

 the Genesee division. 



The smallest division of the formation scale is the stage. 

 This may comprise a number of strata, as in the case of the 

 Hamilton stage. It ma^^ be subdivided into smaller forma- 

 tions, as is the case in the example here cited. The names 

 applied to the stages are commonh^ locality names, but as in 

 the Corniferous stage, the lithological character may furnish 

 the name. The strata comprised within a stage are usually 

 restricted in distribution, seldom covering more than a few 

 hundred square miles of area, and they were all deposited 

 during the continuance of the corresponding epoch. The 

 stages are united into groups, the groups into series, and the 

 series into systems. Groups are local in distribution, their 



