HORIZONTAL SECTIONS. 63 



from which all the heights marked on the ordnance maps 

 have been calculated. But in some instances it is found 

 more convenient to work from an imaginary horizontal 

 line, as datum, so many feet above or below this stand- 

 ard ; in others, to take a fixed point on some permanent 

 structure, as a church, or a bridge, and to commence 

 therefrom an imaginary datum. In geological sections, 

 which frequently shew the strata far beneath the sea- level, 

 it is usual, as in those published by the Government 

 Survey, to assume a datum, " 1000 feet below the level 

 of the sea." 



Bench marks. Here and there along a line of section, 

 or elsewhere, a series of intermediate points are selected, 

 as " bench marks," for the sake of more convenient local 

 reference at any future time. The heights of these 

 points above or below datum are known from the ord- 

 nance levelling, from our own observations, or otherwise, 

 and their positions are indicated or described. All the 

 points on the ordnance maps, where heights are figured, 

 are so many bench marks, the levels of each having been 

 carefully ascertained, and the result checked by re- 

 petition and calculation. 



It is not always necessary, nor is it always possible, 

 to obtain a B. M. of known height above, the sea, from 

 which to commence a proposed line of geological section ; 

 in such cases a" fixed local starting-point must be selected. 

 If near home, as good a B. M. as any is the step of one's 

 front door, and for aneroid observations, perhaps frequently 

 taken for other purposes than a continuous section, none 

 could be better. While running a section line, started 

 from a local B. M., we may possibly come across one of 

 the ordnance heights ; this will enable us, if desired, to 



