
278 STRUCTURAL AND FIELD GEOLOGY 
saved the trouble of determining heights which, for various 
geological purposes, it is often necessary to ascertain. 
When large maps like those referred to are not available, 
and the observer has to content himself with maps on a much 
smaller scale, he may occasionally be compelled to redraw 
portions of his map on a larger scale. Such will be the case 
when the geological structures are so highly complicated that 
they cannot be indicated save in a generalised way on a 
small map. Every field geologist’s note-book is sure to 
contain enlarged sketch-maps of this kind, showing in detail 
complex structures which it would be impossible to represent 
upon any ordinary topographical map. And such enlarged 
portions of his map may serve subsequently as illustrations to 
accompany the observer’s monograph or paper descriptive of 
the region surveyed. 
The maps of some countries which are only sparsely 
settled are often little better than generalised sketches, 
making no pretensions to accuracy; while the topography 
of many wide regions has not yet been delineated even in 
outline. Geologists in such cases must be prepared to do 
some topographical surveying for themselves if they wish to 
prepare a geological map. In several of our colonies 
surveying of this kind has been carried on by geologists 
concurrently with their own special work. Students of 
geology, therefore, if they intend emigrating, should certainly 
acquire some knowledge of topographical surveying before 
leaving home. Even if they have relatively accurate maps 
provided for them, they may yet frequently find it necessary 
to correct these or to lay down the topography in greater 
detail. 
Geological Data to be Mapped.—Assuming that the 
student begins his field work in this country, he has, of 
course, accurate and detailed maps at his service, which is 
a very great advantage: for it will readily be understcod 
that when the topography is inaccurate the geological lines 
cannot be otherwise than distorted. An approximately 
perfect geological map must, therefore, in the first place, be 
thoroughly accurate as regards its topography. It should 
also be on not too small a scale, for the larger the map the 
ereater the detail that can be shown, and the more readily 
