468 APPENDIX. 



faces, when double the labour, in the chisel form, re- 

 places but the original two. 



II. On the Construction of Geological Maps. 



If I point out a few rules for the construction of 

 geological maps, experience will soon teach where 

 they may be modified or improved. On a black map, 

 it is impossible to render intelligible a numerous or 

 intricate set of rocks, by any system of colouring; and 

 hence a more vacant duplicate may become necessary. 

 It has been proposed to associate certain colours with 

 specific rocks, as their permanent emblems : but no 

 project for this end has fulfilled the conditions, as it is 

 in reality, impracticable. In such a hieroglyphic lan- 

 guage, there ought to be a sufficient number to allow 

 one for each rock, each ought to be readily distin- 

 guishable, however approximated,, intermixed, or mi- 

 nute, and each should also be immediately referable to 

 the key. They should further admit of being asso- 

 ciated in groups, indicating those of the rocks them- 

 selves ; while, to unite harmony of effect with this, 

 would complete every thing. Something must there- 

 fore be surrendered : and, this is, all hope of a distinct 

 tint for each rock under every circumstance of approx- 

 imation and extent, so that every mass will require 

 its own key; while this is fully compensated by the 

 advantages retained, and the resulting clearness. 



Though the distinct tints are numerous, not many 

 can be distinguished on an engraved map, under their 

 frequently minute spaces and intermixtures ; while, to 

 increase them by dots or hatchings, is only practicable 

 on a pale map and a large space. But I need not 

 give a catalogue : remarking only, that it is conve- 

 nient to reserve the intense and the opaque colours 

 for the smaller spaces. Experience must still be the 



