HORIZONTAL SECTIONS. 61 



'-J JVUA Jil 



UNIVERSITY < 



CHAPTER IIJl 



CALIFORNIA 



HORIZONTAL ~ 



Datum-level Bench-marks Levelling, by Aneroid, by Level 

 Level-book Plotting, from heights Levelling, by Theodolite 

 Level-book Plotting, from angles Filling in Geology 

 Apparent dip. 



IN constructing a geological section it is most important 

 to have the surface of the ground along the line of such 

 section accurately represented ; for if not, error is sure 

 to creep in, where perhaps least expected, as might easily 

 happen in such an instance as this. A road runs along 

 the line of a proposed section at one part A, a pit by 

 the roadside exposes a limestone 1 foot thick, with shale 

 above and below; by the clinometer it is found that this 

 bed is perfectly horizontal. The road appears to the 

 eye to be horizontal also, but in reality it rises at the 

 slight angle of half a degree. A mile farther on another 

 pit, similar to the first, shews a limestone 1 foot thick, 

 with shales above and below, just the same as at A, also 

 horizontal. The beds seen in the two pits are surely 

 identical, at least it seems natural to come to that con- 

 clusion ; but in reality the bed seen at A is here at B, 

 owing to the slight rise, actually from 40 to 50 feet 

 beneath the surface. It might even happen that the 

 higher of two beds would be shewn passing under the 

 other were the surface judged merely by the eye ; there- 

 fore, if error is to be avoided, the section must be drawn 

 from ascertained data or by the aid of some instrument. 

 There are many means by which a line can be drawn 



