CHAPTER VIII 



THE TRACING OF GEOLOGICAL BOUNDAI 



WHETHER or not the observer sets about the construc- 

 tion of a map, he can form but a limited notion of the 

 geology of a country if he confines his attention merely 

 to a few quarries or lines of natural section. Having 

 learned in such openings what is the nature and order of 

 succession of the rocks, he ought to try to follow them 

 thence into other parts of the country, and in so doing, 

 endeavour to note as he goes any variation in character 

 which they may present, and to take account of every 

 topographical feature which may serve to indicate the 

 disposition of the rocks below. 



A very short experience of geological work in the 

 field suffices to show the observer that over wide spaces 

 he cannot actually see what rock lies beneath him. He 

 may get an admirable section laid bare in some ravine or 

 brook, or by the shore of the sea ; but beyond the limits 

 of this section the ground may be deeply buried under 

 vegetation, soil, sand, gravel, clay, or other superficial 

 formation, and no other section may occur for an interval 

 of, it may be, several miles. Yet he must form some con- 

 clusion as to the nature of the rocks between these \>' 



