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NEW YORK STATE MUSEUM 



amples of the effect of the dragging pressure and the percussive 

 force of the boulder-shod ice. The rock is too hard to accept much 

 furrowing or mass removal on the flat surfaces, but its brittleness 

 favors the production of fractures due to compression and to strik- 

 ing force. Of these features two classes will be briefly described. 

 The hard boulders held as planes and hammers in the bottom ice 

 have produced two kinds of curving fractures, one class convex up- 

 stream or toward the boulder, the other convex downstream or con- 

 cave toward the boulder. Those with the concavity facing down- 

 stream, that is to say, with the convexity toward the producing tool, 

 fall under the category of " cones of percussion " or " chatter 

 marks." Many excellent examples of these concentric fractures are 

 seen, some of large size or up to 10 inches of arc and forming from 

 one quarter to one third of the circle. Sometimes the parallel con- 

 centric fractures are closely crowded, several within an inch, but 

 are usually somewhat more open, three or four or less to the inch. 

 Figure 13 is traced from a " rub " taken by the road near the house 



Six Inches 



Fig. 13 Chatter fractuies 



of William Northrup, 3 miles northwest of Redwood and 3 miles 

 east of Alexandria Bay, and about Y\ of a mile northeast of the 

 curved scorings described above. In this case 11 fracture lines lie 

 within 4 inches along the axis of the curvature, most of them being 



