,l ' .1..M.I. M. MM- 39 



crating each stream at the Mine points on each map. 



l>l.uk line is i winding one, in I- 

 nearly straight Should this boundary be a line of dis- 

 location, the reader will see that by the one map be 

 might be led to speculate upon a sinuous dislocati< 



In Kurope, geologists can generally obtain accurate 

 maps on various scales, which, by supplying a good 

 topographical basis, enable them to reach any degree 

 md completeness in geological map -making. 

 It is useful, however, to be able to construct one's own 

 rough field -map, or to correct a faulty one. For this 

 purpose we avail ourselves of the ordinary methods 

 of thangulation. We may measure, as accurately as 

 cable, a base-line along some level piece of ground, 

 such as a river-meadow or a sea-shore. From each end 

 of our measured line we take a bearing with an azimuth 

 compass to some neighbouring object The point of 

 intersection of the lines of these two bearings gives the 

 position of the object on the map. Having one or two 

 triangles constructed in this way, we may continue 

 triangulating the whole district and filling in the 

 topography, so as in the end to produce a map which, 

 though not quite accurate indeed, will probably serve 

 our immediate purpose. 



In those parts of the world where no good maps yet 

 exist, geological and topographical sun-eying are some- 

 times conjoined I may cite, as admirable illustrations of 

 this union, the explorations of the river-courses of Canada 

 William Logan, first Director of the Canadian 

 Geological Survey. He and his colleagues had to furnish 



