aiz Ol'TMMs,,! ] n I.D-GEOLO(,V FART n 



a> in the Ordnance Sui\ey mtpt, a section may be 

 drawn by using the contour li 



The more clearly a geological map represents the 

 structure of a country, the less need is there for any 

 additional explanation, so that a perfect map. 

 in scale and detailed in execution, should he marly 

 independent of sections or other assistance, excq 



which cannot be expressed upon a map. But such 

 a map can comparatively seldom be made, and dearly 

 constructed sections always save much time and labour, 

 as they enable the structure of a region to be seen and 

 comprehended almost at a glance. \Ve must usually be 

 content with a map on a small scale, an imperfect topo- 

 graphy, and other defects, which compel us to supplement 

 the map with lines of section, so drawn as to convey to 

 the eyes of others exactly what we have ourselves seen, or 

 believe, to be the geological structure of the district or 

 country in question. 



A section may either be horizontal or vertical, that is, 

 it may show what would be seen if a deep trench could 

 be cut across hill and valley, so as to expose the relations 

 of the rocks to each other, or it may exhibit the arrange- 

 ment and thicknesses of the rocks as they would appear 

 if we could pile them up into a tall column one above 

 another in their proper order of succession. On a small 

 scale, Figs. 33 and 64 may be taken as examples of vertical 

 sections. This kind of section is chiefly of use in detailed 

 work, as, for instance, among coal-fields, where the 

 various strata of one pit or part of a district are to be 

 compared with those of another, or in localities like the 

 coast-sections of the Tertiary rocks of the Isle of Wight, 



