STRATIFIED ROCKS. 73 



from such source determines the relative size of the particles 

 making up the sediment. The source may be far up in the 

 interior of the continent where river erosion or lake erosion 

 is eating away the land, or it may be on the ocean-shore, 

 but in general it is true that local geographical conditions 

 are fundamental in determining the lithological character of 

 geological formations. 



Varying Conditions of Environment in Relation to Time- 

 estimates. The conclusion from these observations is that all 

 sedimentary rocks may be supposed to have been formed 

 within about a hundred miles of the shore from which the 

 sediments were derived. This theory is supported by the 

 deep-sea soundings, which show very small amount of mate- 

 rial accumulated on the bottom of the present ocean at great 

 distances from land. From these considerations we turn to 

 our classification of formations, and see why it is that we 

 cannot expect to find uniformity of details in either the 

 structural or stratigraphical order, or in the lithological 

 composition of the formations, (i) At the same time there 

 may be in process of formation a limestone, a sandstone, a con- 

 glomerate, and a mud-shale, and all may be forming within 

 a relatively short extent of coast. (2) In the same period 

 of time the thickness of material accumulated may greatly 

 vary ; while an inch of limestone may be deposited in one 

 place, a hundred feet of sandstone may be formed in another. 

 Thus the limestone of one locality may be represented by a 

 sandstone in another, and a thousand feet of strata in one 

 place may be represented by a hundred or less in another not 

 far distant. 



Relative Order of Deposits in Relation to Depression and 

 Elevation Another series of facts may be considered in this 

 place. The shore-lines do not remain constantly the same for 

 the accumulation of sediments. The simple fact that there 

 are marine fossils in rocks above the level of the ocean is 

 evidence that there has been a change in the relative level 

 of land and ocean surfaces ; there has been an elevation of the 

 land surface. Since the conditions of sedimentation vary 

 with the distance from shore-line, a particular series of these 

 conditions extending from shore-line out into deep water will 



