GEOLOGY, 287 



trustworthy topographical map on a sufficiently large scale to 

 admit of the more important geological details being entered 

 upon it This map may either be made by the geologist and his 

 .assistants, or by previous topographers. In new countries, such as 

 the unexplored tracts in North America, it is customary to com- 

 bine ordinary geographical surveying with geological exploration, as 

 in the surveys of Sir William Logan in Canada, and Dr. Hayden 

 among the Rocky Mountains. In long-settled states the produc- 

 tion of a good topographical map has for the most part preceded 

 sometimes by a long interval the preparation of a geological one. 

 In Great Britain the Government Geological Survey makes use of 

 the maps of the Ordnance Survey on the scales of i-inch and 

 6 inches to the British statute mile. Provided with his map 

 as a groundwork for his own labours, and not encumbered with 

 any work properly devolving on the topographer, the geologist is 

 not burdened with many surveying instruments. His most useful 

 tool is the hammer, which, for the purposes of mapping, ought not 

 to be too heavy ; its weight and shape should be so regulated as 

 to secure that just sufficiently large pieces can be broken even 

 from weathered surfaces, to show the true internal character of the 

 rock. He must carry also a small pocket-lens for detecting the 

 minuter textures and crystals of rocks and structure of fossils ; 

 a clinometer for measuring the angles of inclination of strata with 

 .the horizon, the most convenient forms being those which are 

 most portable and least liable to get out of order ; an azimuth 

 compass for taking bearings and fixing the direction of inclination 

 of strata, the line of faults, dykes, &c. A small protractor to 

 enable him to place these observations accurately on his map ; a 

 series of pencils to mark by different colours and signs the position 

 of various formations on the map, and a note-book to contain 

 detailed descriptions written on the spot, and too lengthy to be 

 put on the map. These few portable instruments, which need 

 not be visible on the person of the travelling geologist, suffice to 

 enable him to accomplish the most complex geological surveying. 



