UNSTRATIFIED AND STRATIFIED ROCKS. 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 

 unstratified rocks. 



The unstratified rocks are also often called primary 

 rocks, because they occur below the others. These 

 rocks are the granites, syenites, traps, etc. They have 

 no arrangement into regular strata, but are confused 

 crystallized masses, evidently the result of fusion; 

 they have 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- 

 nites, and traps, or basalts, which cover so much of 

 the earth's surface. 



The stratified rocks may be divided into secondary, 

 and tertiary, formations, according to their age. The 

 primary rocks, as has been stated, bear marks of fu- 

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

 the secondary, and tertiary rocks. Their materials 

 have evidently all been deposited by water, having in 

 many cases undergone striking changes afterward, 

 but always retaining marks of their origin. Some- 

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

 limestones; sometimes thin, like the leaves of a book, 

 as in some slates. 



a. An example of stratification may be seen in 

 almost any sand or clay bank, where the successive 

 deposits by water are clearly marked; some of the 

 layers being quite thick, others very thin; some quite 

 level, and others again very undulating. 



These strata were of course all deposited in regular 

 succession, one above another : if there had been no 

 subsequent changes, then we should only be acquaint- 



