GEOLOGY. 289 



depicted as they might appear if deep shafts or long cuttings were 

 made in the ground beneath us. Such sections are either hori- 

 zontal or vertical, the former showing the geological structure of 

 districts, the latter indicating the order of succession of the rocks 

 at particular places. 



Horizontal sections are for the most part generalised diagrams 

 to show the ascertained or inferred arrangement of the rocks 

 between given localities. It is only when they are drawn on a 

 true scale that is, when the height and length are expressed on 

 the same scale that they attain their highest excellence. For low 

 countries of very simple geological structure, especially where the 

 rocks are flat, the adoption of a true scale may not be desirable ; 

 but for hilly or mountainous ground, where the rocks have been 

 thrown into many different positions, and where the external forms 

 of the surface bear close and evident relation to the internal struc- 

 ture, the preparation of sections on a true scale where practicable, 

 cannot be too strongly recommended. Illustrations of the appli- 

 cation of this rule are supplied by the horizontal sections of the 

 Geological Survey of the United Kingdom on the scale of six 

 inches to a mile. These are either levelled on the ground by the 

 officers of the Survey, or plotted from the previous levellings of 

 the Ordnance Surveyors. 



Vertical sections either express in a general way the ascertained 

 or inferred succession of formations in a district, or the actual 

 measured subdivisions at a particular locality, as in a coal pit. 

 For the sake of distinctness they are usually drawn to a consider- 

 ably larger scale than the horizontal sections. It may be added 

 that such further details as cannot be inserted either on map or 

 sections are extended and printed in memoirs or other explanatory 

 text. 



It remains to notice the means at present available towards the 

 illustration of geological science for the purposes of teaching. 

 The collection of typical minerals, rocks, and fossils above spoken 

 of, are here of essential service. They may be so selected, pre- 

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