306 G. K. GILBERT — -CRESCENTIC GOUGES ON GLACIATED SURFACES 



was first made. The oblique, or conoid, fracture may therefore be re- 

 garded as the primary product of the causative force, and the vertical 

 fracture as secondary; and in seeking a cause of the phenomenon I have 

 given first attention to forces which might be appealed to in explanation 

 of the conoid fracture. 



There is another conoid fracture with which geologists are familiar, 

 the fracture often made in obsidian, or other homogeneous brittle rock, 



by a light blow of the hammer. 



This is sometimes called the conoid 

 of creseentic ofpercussion(seefigure4). Usually 

 it circles completely about its axis, 

 a to b or but sometimes it is one-sided. Its 

 relation to the surface struck by th e 



Figure 



3. — Cross-section 

 Gouge. 



The section represented is from 

 C to D of figure 2. 



hammer resembles closely the relation of the glacial conoid to the ex- 

 ternal surface of the bed-rock, and the one fracture may help to explain 

 the other. The conoid of percussion is caused by a blow ; that is, by the 

 instantaneous application of pressure to a small area. N"o way has 



Figure 4. — Diagrammatic Sections of Fractures. 



The horizontal and vertical lines represent the top and side of a mass of rock ; PP, 

 conoid of percussion, the causative blow being struck in the direction indicated by the 

 arrow ; G, conchoidal fracture, with arrow showing direction of blow ; G, conoid and 

 vertical fractures of creseentic gouge, with oblique arrow showing theoretic direction of 

 causative pressure. 



occurred to me in which a glacier can make such a fracture by means 

 of a blow, but it seems possible, as I shall presently explain, that a glacier 

 can slowly apply considerable differential pressure to a very restricted 

 area ; and there is some reason to believe that suddenness of impact is not 

 essential to the production of conoid fractures. The ordinary con- 

 choidal fracture, which is a near relative of the conoid of percussion, is 

 commonly developed in brittle rock by a blow struck near an edge (figure 

 ■i) ; but it is also produced by simple pressure in the manufacture by 

 Indians of flint and obsidian implements. 



In a single instance a conoid fracture in granite was observed to circle 

 completely around its axis (see plate 38), thus simulating still more 



