216 



OUTLINES OK I II Ll> 



r \K i ii 



to erect our verticals for the heights. Having obtained 

 the correct measurements of the surface from our 

 own levellings, or those of other surveyors (in Britain 

 the contoured maps of the Ordnance Survey ai 

 valuable for this purpose) we proceed to mark off from 



I51?8 I I I I I 



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FIG. 87. Stages in the construction of a geological section. 



our datum-line a series of points, the height of each of 

 which is known. How this is done is shown in Fig. 

 87, A. A line is then drawn, connecting all the points 

 together, which gives, it will be observed, the general 

 contour of the ground. To ensure greater fidelity of 

 detail it may be well to walk again over the ground with 

 the plotted section in hand, so as to be able to fill in 

 any little inequalities of surface, and at the same time 



