I50 ON COLOURING GEOGNOSTIC AL MAPS. 



followed. But it is liable to some objections ; for 

 example, it only points out the different rocks ; 

 it does not make us acquainted with their geogno- 

 stic relations ; and the colours hitherto chosen to 

 represent the various rocks, are so very different 

 from each other, that maps coloured in this man- 

 ner have a disagreeable and patch-like appearance. 



In colouring geognostical maps, it is absolutely 

 necessary to establish and follow certain rules, in 

 order that there may be an uniformity or unity 

 of execution in all the parts, and consequently 

 that similarly executed delineations, may appear 

 as parts of one and the same whole. The fol- 

 lowing rules should be observed in colouring 

 maps: 1. In every case we must use such colours 

 as will allow the ground-work of the map or deli- 

 neations of the mountains, to appear through them 

 distinctly. 1, The colours must agree as nearly 

 as possible with nature, that is, they must corre- 

 spond with the most common colour of the rock, 

 or at least differ from it as little as possible, and 

 agree with the transition suite of the colours. 3. 

 The use of all very bright colours must be avoid- 

 ed. 4. The colours must not be too pale or 

 too deep, and they ought to be laid on as much as 

 possible of the same intensity : perfectly dark and 

 light coloured rocks are exceptions to this rule,, 

 &• The colours of mountain-rocks must form 

 suites or transitions, in order to express the transi- 

 tions of the rocks into each other : at the same 

 time, they must be sufficiently distinct from each 



