. Bulletin of the Geological Society of America. Geology. 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. T
. Bulletin of the Geological Society of America. Geology. 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 the 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. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Geological Society of America. [New York : The Society]
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