CHAPTER IX 



THE UNRAVELLING OF GEOLOGICAL STRUCTURE Ml , 

 STRIKE, OUTCROP 



IF we could only recognise rocks where they are actually 

 seen, but form no satisfactory conclusion regarding their 

 distribution under a concealing mantle of vegetation or 

 superficial detritus; if we could tell the arrangement and 

 measure the thickness of strata only at the surface, but 

 offer no opinion as to the prolongation of these strata 

 underground, we should never know much about the 

 crust of the earth, and certainly could do comparatively 

 little to advance geological inquiry. Fortunately it is 

 not only possible but comparatively easy to pronounce 

 upon the subterranean arrangement of rocks from in- 

 dications obtainable at the surface. We seldom need to 

 bore or dig. It is usually enough if we can avail our- 

 selves of the surface evidence, and gain from it informa- 

 tion respecting the probable arrangement of the rocks 

 below, or in other words, the geological structure of 

 the ground. How this is done, let us now proceed to 

 consider. 



Horizontal Strata Outcrop. In a region where the 

 rocks are all horizontal, only the uppermost stratum may 



