SUBDIVISIONS IN GEOLOGICAL HISTORY. 399 



stone in New York and Pennsylvania is of cotemporaneous origin with a 

 limestone in the Ohio and Mississippi valleys. Some rocks in eastern New 

 York are not found in the western part of that state, and some in the central 

 and western part not in the eastern. 



2. In all periods, sand-beds, mud-beds, clay-beds, pebble-beds, and lime- 

 stone-beds have been simultaneously in progress over different parts of the 

 globe ; and, if a period is known in geology as solely a period of limestone, 

 it is because science has not yet discovered where the beds of sand, mud, 

 or pebbles were being deposited while the limestone was making over its 

 regions. The idea of a period of sandstone-making, or of limestone-making, 

 is therefore an absurdity ; for sand deposits are local ; a short distance off, 

 there may have been, in all times, as now, mud deposits. Still, it is true 

 that, over continental seas, the prevailing depositions have sometimes been 

 of limestone material, and sometimes of mud or sand; yet this has been true 

 for certain great regions in the seas of a continent, rather than for all its 

 seas at once. 



3. Again, a stratum of one era may rest upon any stratum in the whole 

 of the series below it, — the Coal-measures on either the Archaean, Silurian, 

 or Devonian strata ; and the Jurassic, Cretaceous, or Tertiary on any one of 

 the earlier rocks, the intermediate being wanting. The Quaternary in 

 America in some places rests on Archaean rocks, in others on Silurian or 

 Devonian, in others on Cretaceous or Tertiary. 



4. In addition, denudation and uplifts have thrown confusion among the 

 beds, by disjoining, disarranging, and making complex what once was simple. 



Amidst all these sources of difficulty, how is the true order ascertained ? 



Means of correlation. — The following are the means employed : • — 



1. Order of superposition. — When strata are little disturbed, vertical 

 sections give the true order in those sections ; and so also may outcrops of 

 inclined strata over the surface of a country. In using this method by 

 superposition, several precautions are necessary. 



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

 folded upon one another, so as to make an upper layer a lower one (see page 

 104), — a condition to be suspected in regions where the rocks are much 

 tilted. 



Precaution 2. — It should be seen that the strata 350. 



under examination are continuous. A fault in the 

 rocks may deceive ; for it makes layers seemingly 

 continuous which are not so. Faults are common in 

 regions of upturned rocks and may occur when the 

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

 part of a bluff (as ah, Pig. 350) have settled down 



bodily (c) to the bottom, so as to seem to be continuous with the older ones 

 of the bottom (as c with d). In other cases, caverns in rocks have been 

 filled through openings from above, and the same kind of mistake made. 



