PART V. 



CHAPTER I. 



Survey of Older Eocks Boundaries and Faults Downthrow 

 Vertical Section Eruptive Eocks Veins. 



Survey of Older Rocks. As an example of geological 

 mapping which involves the application of nearly all 

 the methods and operations described in the preceding 

 pages, we may now take a district supposed to consist 

 of palaeozoic rocks. These being much older than 

 those previously mapped (pp. 17 to 27, and figs. 5 8) are 

 consequently much more likely to have been disturbed, 

 faulted, and contorted. In mapping such rocks it is a 

 good plan first to make oneself well acquainted with 

 the physical geography of the district to be surveyed 

 to find out the watershed of the area, the system of its 

 valleys, the highest points of its hills, and the amount 

 of their elevation. We get therefrom a general idea of 

 how the rocks, if not eruptive, will probably be found 

 to run, for, as stated in Part I. (p. 15), beds almost in- 

 variably dip towards higher ground to the form of 

 which their inclination has indeed greatly contributed ; 

 therefore the contour of a hill approximately indicates 



