On Constructing Geological Models. 



201 



Back-sight. 



Pore-sight. 



Rise. 



Fall. 



Reduced 

 Level. 













100-00 



Above datum. 



10-56 



4-35 



6-21 





106-21 





8-21 



3-74 



4-47 





110-68 





2-93 



6-49 





3-56 



107-12 





1-87 



13-72 





11-85 



95-27 





and used, by entering in the first column the reading taken 

 upon the staff at some ascertained point (which, to avoid 

 minus quantities, is usually taken one hundred feet above 

 an imaginary point below) ; then in the second column another 

 reading of the staff, after it has been removed higher or 

 lower, as the case may be, the difference between these will 

 manifestly be the difference of level between the two positions 

 in which the staff has rested, which will indicate rise or fall, 

 and which, added to or subtracted from the reduced level, 

 will ascertain the elevation of the last spot where the staff has 

 been placed. In the above instance this would be 95*27 

 above datum. We thus ascend or descend twenty-five feet 

 from the starting point, and then commence a contour, and as 

 we proceed, transfer it, as nearly as possible, guided by fences 

 and other landmarks, to the tracing. This contour may be 

 taken either by moving the levelling staff up or down as we 

 proceed, till the very same figure is observed on it as we 

 started with — or by using the levelling book as before ; only 

 instead of ascending or descending we keep to as nearly as 

 possible the same elevation, always taking care to register the 

 figure observed with great care, whenever we move the level, 

 and again when the staff itself is moved. The same operation 

 is repeated at heights of 50, 75, 100 feet, and so on, so that at 

 last our map is covered with lines after this manner. 



The next step will be to reduce this map and its fines to 



