GEOLOGICAL MAPPING, 13 



part of its length into feet, on the scale of 6 inches to 

 a mile it will be serviceable for plotting or measuring 

 from sections (fig. 2). 



Tracing Boundaries. A geological ma*p is one which 

 defines the area occupied by the denuded edge, or upper 

 surface of each formation, where it comes to the level of 

 the ground. To accurately construct such a map, therefore, 

 every part of the ground must be more or less minutely 

 examined. If by any means, as by boring or otherwise, 

 the surface of an area were to be proved at say every 

 100 yards and the varying results shewn by different 

 colours, a geological map would be roughly presented. 

 But it would be a,n approximation only, for there would 

 still remain to be shewn the exact position between the 

 borings where the lines of division run. In Chapter II. 

 it is intended to explain how such lines may be traced 

 and represented on the map to be geologically tinted. 



Note. In speaking of "boundary lines," those 

 are meant which bound a formation, which describe 

 its lower margin, and, in fact, indicate its extreme 

 occurrence in any direction. Its upper edge, where 

 it first appears at the surface, is called its " line of 

 outcrop," and this of course corresponds to the 

 boundary line of the overlying formation. 

 If a certain set of fields on one side of a road, fence, 

 or brook, shewn on the map, were entirely on one forma- 

 tion, and another set of fields on the opposite side were 

 entirely on another formation, then the engraved line 

 would answer also for that of the geological division. 

 But it rarely happens that the arbitrary lines of a road 

 or fence follow the intricate windings of a natural divi- 

 sion of the rocks. With a brook the case is somewhat 



