UNSTKATniED IND STKATIPISD KOCXS. 199 



The labors of geologists have resulted in the es- 

 tablishment of certain great divisions, among the 

 rocks which present themselves for our inspection. 

 The leading, and grand division, is into stratified, and 

 unstratihed rocks. 



The unstratified rocks are also often called primary 

 rocks, because they occur below the others. Theae 

 rocks are the granites, syenites, traps, etc They hare 

 no arrangement into regular strata, but are confused 

 ( ■ /»d masses, evidently the result of fusion; 



t. . all at one time been melted like lavas, and 



are, in fact, ancient lavas, which, in cooling, have 

 assumed their present form. Occasionally these old 

 lavas have burst up through the stratified rocks, just 

 as volcanic eruptions do now, and have cooled in the 

 open air : in such places we have the ranges of gra- 

 [ji'rs, and traps, or basalts, which cover so much ci 

 {:.■ < arth's surface. 



The stratifi. ■ ' Ilvided into secondary, 



and tertiary, i uig to their a^e. The 



primary rocks, as has been stated, bear marks oi fb- 

 sion, and of having been formed by heat; not so with 

 the secondary, and tertiary rocks. Their materials 

 have evidently all been deposited bv water, having in 

 many cases undergone adriking changes aftemrard, 

 but always retaining marks of their origin. Some- 

 times the strata are thick, as in some sandstones, and 

 Umeitonet; scmietimes thin, like tlie letTcs of a book, 

 as in some dates. 



a. An example of stratification may be seen in 

 abnost any sand or clay bank, where the succeasiTe 

 dieposits by water are clearly marked; some of tike 

 layers being quite thick, others very thin; some quite 

 level, and others again very undulating. 



These strata were of course all depoKtted in regnlat 

 suc ce s si on, one above anotlicr : if there had been no 

 subsequent changes, then we should only be aoquaial- 



