ON COLOURING GEOGNOSTICAL MAPS. 157 



Thus, a bed of greenstone, in inclined strata of 

 mica-slate, as represented in the map, will have its 

 lower side marked with a broad dark-green co- 

 loured line, but its upper side with a broad line of 

 dark pearl grey. 



Veins are to be represented by lines drawn in 

 the direction of the veins of the district. Metal- 

 liferous veins should be pointed out by red lines ; 

 and veins rilled with mountain-rocks, by lines of 

 the same colour as the rock of which they are 

 composed. 



There is still another object to be attended to, 

 in the execution of geognostic maps : it is the 

 dip of the strata. This is to be expressed by 

 black- coloured arrows, whose length should be in 

 proportion to the angle of inclination, and their 

 direction to the point of the compass towards 

 which the strata dip or incline. When.the strata 

 are vertical, or under any angle from 90 0 to 8o Q , 

 they are marked by two cross lines thus x ; hori- 

 zontal strata, or strata under any angle from o to 

 io°, by two lines crossing each other at right 

 angles, and having darts at each extremity. The 

 intermediate angles, from 80 ° to 10% are marked 

 by simple arrows, which are ~, \, and ~ of an 

 inch in length. The arrow -J inch in length in- 

 timates, that the strata are inclined at any angle 

 between 8o° and 6o° : the arrow -J inch in length 

 that the strata are inclined at any angle between 

 60 0 and 40 0 : the arrow ~ inch in length, that the 

 strata are inclined at any angle between 40 0 and 

 jo°. The first, or shortest arrow, is said to point 



