CHAPTER XVIII 



HISTORY FROM THE ROCKS 



AN inference of great importance can be drawn from the occur- 

 rence of lamination in a stratified rock. Each layer is made 

 of denuded material spread out on a sea bed. It in turn is the 

 sea bed, on which a new layer is spread out. Thus a stratum 

 made up of many laminae must preserve the deposits of many 

 successive sea beds resting one upon the other. It will contain 

 the history of a considerable period of quiet deposition under 

 water. In order to read this history in chronological order, it 

 will be necessary to begin at the beginning, that is, at the lowest 

 lamina ; and this will be the case not only when the beds remain 

 in the horizontal position in which they were laid down, but even 

 after they have been tilted and folded. This important principle is 

 known as the order of superposition, and when it is read correctly 

 the sequence of the laminae gives the sequence of events during 

 which the rocks were laid down. The alternation of coarse and 

 finer laminae, for example, will tell of the successive deepening 

 and shallowing of the sea, just as the alternation of layers of 

 sand and mud on the delta of the Nile tells of a succession of 

 inundations and sand storms. 



Evidently the principle will apply not only to laminae, but to 

 the larger beds or strata. If a bed of clay rests on one of chalk, 

 the latter will be the older, and the interpretation of the rock 

 succession will indicate that a clear sea with deposit of calcareous 

 organisms was followed by a muddy sea, to which rivers or currents 

 swept in quantities of fine denuded matter. The duration of the 

 two periods will be roughly expressed by the relative thickness 

 of the two beds. By the application of this principle to all strati- 

 fied rocks found in juxtaposition it is possible to ascertain the 

 relative ages of them all, and to construct a time scale out of 

 them in which all the strata, such as the Lias, the Coal Measures, 





