848 THE JOURNAL OF GEOLOGY. 



strise are sometimes found to be far from parallel. Less com- 

 monly, this is true for small areas. Divergent and inharmonious 

 as these various directions sometimes seem to be, fuller knowledge 

 discovers a system in their arrangement. In many areas, the 

 strise are found to arrange themselves in systems. Within each 

 system the striae are found to be divergent from a common axis. 

 Such axis is not generally a mountain range, or even a ridge. It 

 is oftener a broad valley. 



The systematic arrangement of the striae, as developed in some 

 regions, is illustrated by the accompanying figure (Figure 5), 

 which represents, in a diagrammatic way, the arrangement of 

 striae on the rock surface beneath the drift of eastern Wisconsin. 

 The striae on the right-hand side of the figure appear to represent 

 the marginal part of a second system, the axis of which is in the 

 trough of Lake Michigan. In other localities, also, two similar 

 systems of striae lie side by side. 



Within any single divergent system there may be local diver- 

 gences from the common direction. In such cases, the diver- 

 gences are almost uniformly associated with local topographic 

 features. The striae often have such a direction as to indicate 

 that the agent which made them had a tendency to go around a 

 hill or ridge, instead of passing directly over it. This tendency 

 of striae to veer round elevations may sometimes be observed 

 about large and abrupt hills, while in the same locality lower 

 hills with gentler slopes do not appear to have influenced the 

 course of the striating agent. 



Striae are not confined to horizontal or gently inclined surfaces. 

 They sometimes occur on steep slopes, on the vertical faces of 

 cliffs, and, occasionally, even on the under side of overhanging 

 rock masses. They sometimes occur in still more anomalous 

 positions. In the face of the high bluff overlooking Cayuga Lake, 

 for example, a horizontal groove in a vertical face is striated on 

 its upper, lower, and interior sides. The groove in which the striae 

 occur retires eis^ht inches into the face of the vertical cliff. 1 



Where striae are absent from the surface of the rock beneath 



'Seventh Annual Report, U. S. Geol. Survey, pp. 170, 173. 



