6 FIELD GEOLOGY. 



out-crop of Coal-seams, for instance it is well to go at 

 once to maps drawn on a six inch scale, although they 

 may be in some respects inconveniently large. For the 

 plain spaces thereon between roads and hedges admit of 

 the frequent notes necessary in such cases being written 

 on the map itself, instead of in a book specially provided 

 no trifling advantage when the size of the map to be 

 carried is taken into consideration. The maps of the 

 Ordnance Survey, especially those issued during the 

 last few years, are as good as any ; in choosing copies, 

 those should be selected which are clear and distinct as 

 regards the engraved lines, but which are light rather 

 than dark impressions. 



Contour Maps. Some maps have marked on them 

 certain lines, the meaning of which it is well clearly to 

 understand ; these are called " contour lines." To the 

 eye accustomed to them, these lines convey at a glance 

 the physical geography or the actual " shape " of a tract 

 of country its hills and valleys, its precipices and 

 ravines and not only in a sketchy or approximate form, 

 but with heights and depths taken from actual ad- 

 measurement. A contour line runs through all the 

 points at which a perfectly horizontal plane at any given 

 height would intersect the surface of the ground ; in 

 other words, if the land were covered with water to a 

 certain height, the margin of the water would be exactly 

 represented by a contour line drawn at that same eleva- 

 tion. These lines are shewn for every 10, 25, 50, or 100 

 feet, according to the scale of the map and the degree of 

 accuracy required. In geological surveying they are of 

 assistance in the drawing of boundary lines, ascertaining 

 heights and making various calculations. 



