288 SCIENTIFIC APPARATUS. 



For the sake of clearness he finds it needful to use upon the map 

 a system of short contractions and signs to express by a few strokes 

 facts and descriptions which would require many words if fully 

 written out, the details being duly entered in his note-book. From 

 time to time the pencillings on the field map are inked over and 

 the different formations are distinguished by washes of colour. 

 The stages in the progress of a field map are illustrated by 

 examples in the collection. When the map of a district or country 

 is completed on a large scale, it may require to be reduced to a 

 smaller size before publication. Thus, in the Geological Survey 

 of Great Britain and Ireland, most of the field maps employed are 

 on the 6-inch scale, which, except for the mineral fields, are 

 reduced and published on the i-inch scale. 



On geological maps, the areas occupied by different rocks are 

 expressed by conventional colours. No general system of colours 

 has yet been adopted ; and, perhaps, in the present state of the 

 science, such a system could hardly be proposed. There would, 

 however, be a great advantage in appropriating certain colours or 

 patterns to particular rocks or systems, and retaining them in that 

 application in all countries. The geological map of any country 

 would thus at a glance be generally intelligible to the geologists of 

 other countries. As matters at present stand, each country or state 

 selects its own colours, so that the study of its maps by strangers 

 involves a preliminary and sometimes troublesome process of 

 unlearning the significance of the same colours elsewhere, and 

 of learning another application of them. Where colour printing 

 can be employed, great clearness, diversity, and multiplicity of 

 tints can be obtained. 



A perfect geological map should be on such a scale, and with 

 such minuteness of detail, as to contain within itself all the data 

 necessary for the understanding of the geological structure of the 

 area it represents. But in most cases the maps are on too small a 

 scale for this purpose. They are therefore supplemented by sec- 

 tions, wherein the arrangement and thicknesses of the rocks are 



