102 LITHOLOGICAL GEOLOGY. 



limestones, sandstones, shales, and conglomerates ; and they occur 

 horizontal and displaced ; conformable and unconformable ; part in 

 America and part in Europe, Asia, and Australia ; here and there 

 coming to view, but over wide areas buried beneath soil and forests. 



Moreover, even the same bed often changes its character from a 

 sandstone to a shale, or from a shale to a limestone or a conglomerate, 

 or again to a sandstone, within a few miles or scores of miles, and 

 sometimes within a few rods ; or, if it retains a uniform composition, 

 it changes its color so as not to be recognized by the mere appearance. 

 In the United States, many a sandstone in New York and Pennsyl- 

 vania is represented by a limestone in the Ohio and Mississippi valleys, 

 — that is, the two were of cotemporaneous origin ; some rocks in 

 eastern New York are not found in the western part of that State, and 

 some in the central and western not in the eastern. 



In all eras, 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 an era is known in geology as solely an era of lime- 

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

 sand, mud, or pebbles of that era were being deposited while the lime- 

 stone was making over its regions. The idea of an era 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. 



Again, a stratum of one age may rest upon any stratum in the whole 

 of the series below it, — the Coal measures on either the Arcluean, 

 Silurian, or Devonian strata ; and the Jurassic, Cretaceous, or Terti- 

 ary 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. 

 And, if so great diversity of condition exists in one country, far greater 

 may be expected between distant continents., 



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 ascer- 

 tained ? 



Means of determination. — It is plain, from the preceding remarks, 

 that the true method cannot consist in grouping rocks of a kind to- 

 gether, as limestones, shales, or sandstones. It is irrespective of kinds, 

 and is founded on a higher principle, — the same which is at the basis 



