174 STRUCTURAL GEOLOGY. 



this gives us a datum point from which to measure displacement, degrada- 

 tion in the region of uplift, and sedimentation in the region of downthrow. 

 For this reason the group is graphically accented in each section. 



Displacement Diagrams. On Plate II is grouped a series of diagrams 

 designed to represent displacement and degradation. Each diagram corre- 

 sponds to and is derived from a section in Plate I, and is composed of three 

 lines: the sea level line and the surface line, which are the same as those in 

 the corresponding section, and a third, a displacement line, which is the 

 accented line of the section, projected in the region of uplift from the observed 

 outcrop of the group which it represents (Point of Rocks Group), to the 

 position it would have in the section had there been no degradation but 

 displacement only. 



We thus take the sea level as the zero from which to measure displace- 

 ment; and by introducing the surface line, the accented line above becomes 

 a zero for the measurement of degradation in the region of uplift; and as 

 this accented line represents the position of the last bed deposited prior to 

 the inception of the upheaval (which is the highest Cretaceous bed), it also 

 forms a zero line from which to measure the sedimentation on the flanks of 

 the upheaval which occurred subsequent to the inception of the displace- 

 ment; or, in other words, a zero line from which to measure Post-Cretaceous 

 sedimentation. Hence in the region of downthrow, with the accented line 

 as the zero, the surface line measures the amount of Post-Cretaceous sedi- 

 mentation, minus an unknown loss by degradation. 



It may be well here to explain the method by which the position of the 

 displacement line was determined. In the topographic survey the altitude 

 of many points on the surface is fixed with a reasonable approximation^ 

 accuracy by the use of the barometer and theodolite. These points are the 

 junction of drainage lines, the summits of peaks, and many other salient 

 and conspicuous topographic features ; and from these, contour lines are 

 drawn by inspection. Hence the topographic map fixes the position of any 

 bed appealing on the surface with all the accuracy necessary for the scale 

 on which these sections and diagrams are drawn. 



Another factor used is the general thickness of the beds. This is deter- 

 mined by measuring them where they outcrop on the flanks of the uplift. 



