5 8 THE STORY OF THE EARTH. 



some localities, while the level elsewhere is un- 

 affected, so that the succession of rocks may vary 

 in mineral character in the disturbed district, 

 with no indication of change in the succession 

 elsewhere. On this account, the history of every 

 part of the earth needs to be told separately from 

 the other parts, for too little is yet known of the 

 detailed events which took place in the successive 

 periods of time, in the different portions of the 

 globe, to piece the parts of the story together 

 into a complete history of the earth. 



CHAPTER VII. 



ORIGIN OF STRATIGRAPHICAL GEOLOGY. 



GEOLOGY originated in observation of the 

 earth's surface, by which records were made of 

 the order and arrangement in which different 

 rocks occur in England and Wales. This knowl- 

 edge is expressed in two laws. The first is the 



law of stratification. It affirms that the rocks 

 which are anywhere exposed on the surface ol 

 the country are usually portions of layers, which 

 rest Successively upon each other. Therefore 

 they rise from beneath each other, in the order 

 of their relative antiquity, whenever they are in- 

 clined to the plane of the horizon. Kvery such 

 layer is a stratum. Strata differ from each Other 

 in relative antujuity, in their mineral materials, 

 thickness, extension, and degree of disturbance 



from the original condition of their horizontal 



OSltion. The law of succession of the la\ 



upon each other is named their superposition. 



