62 FIELD GEOLOGY. 



that shall represent the surface through any number of 

 given points by some approximately and in a sketchy 

 manner, by others every detail is shewn with the greatest 

 accuracy. Among the former are the methods employed 

 in drawing section lines from the published " heights 

 above the sea," from contour maps, and from heights 

 taken by aneroid barometer; the latter, from levels 

 ascertained by the Y, the dumpy or any other level, 

 and from a series of vertical angles observed by means 

 of the theodolite. All proceed on the same principle of 

 ascertaining the heights at various points, near to or 

 distant from each other, according to the degree of 

 accuracy required, and sketching in the line between 

 those points to correspond as nearly as may be with the 

 surface form of the ground. A few brief remarks will 

 follow on the use of the level and theodolite, for geolo- 

 gical purposes. Before the levels for a geological section 

 are run, the beds have generally been mapped, and the 

 line should be laid out as nearly as can be conveniently 

 arranged at right angles to the strike. For approximate 

 levelling the distances apart of the points where heights 

 are taken can be scaled from the map ; for accurate 

 work, measurements by chain are required. 



Datum-level. By whatever means the heights or 

 levels for a section are taken, it is necessary to have a 

 fixed or standard level to commence from, to which 

 reference can at any time be made, and in regard to 

 which all the other heights are calculated. It is usual, 

 as it is most convenient, to refer all heights to the stand- 

 ard, or as it is called, the " datum-level " adopted for this 

 country, that is, the " level of the sea." By this term 

 is understood the level of mean tide at Liverpool, a datum 



