36 OUTLIM - "I I I H.I )-<,! M.OGY PARTI 



n sometimes approximate, boundary- 

 .md with no hint whatever of geological struct mv. 

 whic.h we now regard as one of the chief objects of 

 geological maps. 



A perfect geological map should represent ist. A 

 full and accurate topography, with the form of the sin tu< e 

 and heights in contour-lines, shading, or otherwise. The 

 Ordnance Survey maps of Britain on the scale of six 

 inches to a mile may be taken as an admirable example. 

 2d. All geological deposits, from the most recent to the 

 most ancient, which may occur at the surface in the di.s- 

 trict, with their mutual boundary-lines accurately traced, 

 and the relation of their distribution to the external form 

 of the ground clearly depicted. 3d. The geological 

 structure of the region, that is, the relation of the rocks 

 to each other, their inclination downwards from the 

 surface, their curvatures and dislocations ; in short, all 

 particulars necessary to enable a geologist to appr< 

 the manner in which the rocks of the crust of the earth 

 beneath the region in question have been built up. 4th. 

 Information which may have special economic value, such 

 as the nature and distribution of the soils, the position of 

 available building materials, the direction, thickness, and 

 extent of ores, coal-seams, or other useffll minerals, the 

 best sources of water supply, etc. 



To fulfil these various requirements the map must 

 evidently be on not too small a scale. If the scale is 

 small, the attempt to crowd a great deal of information 

 into the map may result in confusion of detail, and much 

 of the beauty and usefulness of the work may be lost. 

 In such cases it is better, where practicable, to subdivide 



