166 



Subsurface Geologic Methods 



Normally two or three hundred grains are oriented and plotted for 

 each thin section. Upon completion of plotting, the concentration of 

 points is contoured. That is, the concentrations according to percentages 

 are separated by lines having values such as 1, 2, 3, and 4 percent. The 



Figure 63. Detail of crest of fold in figure 62. The scale is parallel to the b axis. 

 The lineation is caused by the intersection of the schistosity and a bedding 

 plane. The joints normal to the b axis are a-c joints. 



area for each percentage is given a pattern, and the greatest density is 

 generally shown in solid black. This procedure completes the orientation 

 diagram except for marking the reference plane, which must be clearly 

 shown. 



Petrofabric diagrams are classed as "elemental" when a single mineral 

 is studied in one thin section, and as "collective" when the plotted points 

 from a number of elemental diagrams are combined into a single diagram. 



