ARRANGEMENT OF STRATA. 103 



of all history, — successiveness in events. The following are the 

 means employed. 



(1.) Order of superposition. — When strata are little disturbed, ver- 

 tical sections give the true order in those sections, and afford valuable 

 information. Or, where the strata outcrop over the surface of a coun- 

 try, the succession of outcropping layers affords a section, and often 

 one of great range. The vertical extent of such a section may be as- 

 certained as explained on p. 99. In using this method by superposi- 

 tion, several precautions are necessary. 



Precaution 1st. — Proof should be obtained that the strata have not 

 been folded upon one another, so as to make an upper layer in any 

 case a lower one in actual position (see p. 97), — a condition to be 

 suspected in regions where the rocks are much tilted, but not where 

 the tilting is small. 



Precaution 2d. — It should be seen that the strata under examina- 

 tion are actually continuous. 



A fault in the rocks may deceive ; for it makes layers seemingly 

 continuous which are not so. Such faults are common, and often ex- 

 tensive, in regions of upturned or much displaced rocks, and may 

 occur when the dip is slight. In some cases, beds forming the upper 

 part of a bluff (as a b, Fig. 114) have settled down bodily (c) to the 

 bottom, so as to seem to be continuous with the older ones of the bot- 

 tom (as c with d). In other cases, caverns in 



. Fig. 114. 



rocks have been filled through openings from b 



above, and the same kind of mistake made. p^v"/ 7 ^ 

 When the continuity can be established, the t=-^>^^^ 

 evidence may sometimes lead to unexpected pgjjjp|l§lL__^_^ 

 results. For example, it may be found that mM m&se* 



a coal-bed, followed for some miles to one side Lli '"•'•'•' :: - '■•'•'••"• •••jl^-j 

 or the other, is continuous with a shale, and 



both are actually one layer ; that a sandstone is one with a limestone 

 a few miles off; that an earthy limestone full of fossils is identical 

 with a layer of white crystalline marble in a neighboring district ; or 

 that a fossiliferous shale of one region is the same stratum with the 

 mica schist of another. 



Precaution 3d. — Note whether the strata overlie one another con- 

 formably or not. 



Precaution 4tk. — Remember that, where one bed overlies another 

 conformably, it does not follow necessarily that they belong to consec- 

 utive periods, as has been above explained. 



The criterion mentioned, unless connected with others, gives no aid 

 in comparing the rocks of distant or disconnected regions. For this 

 purpose, other means must be employed. 



