174 STUDIES FOR STUDENTS 



correspond in direction with earth movements recorded in adja- 

 cent strata; (5) when the number of striated pebbles in different 

 parts of the breccia varies directly with the amount of shear; 



(6) when the striae on faulted pebbles end at the fault plane; 



(7) when the striated surface is covered with films deposited from 

 solution. Several of these diagnostics are mentioned by Marr 1 

 as characteristic of scored pebbles having the form of glacial 

 bowlders in an English breccia. Tectonic breccias often display 

 slickensided blocks, 2 but they are hardly liable to be confounded 

 with glaciated pebbles. It may be added that the proportion of 

 glaciated pebbles in iceberg breccias may be exceedingly small in 

 comparison with that in the drift-sheets of far-traveled continental 

 glaciers. 



Iceberg breccia, as well as any other, may be sheared after its 

 formation. In this case neither slickensides nor glaciation can be 

 used to disprove the other process. A dual origin seems to be indi- 

 cated in scored pebbles of some of the Permian breccias of England, 

 but there are students who claim that they are due to earth 

 movements only. 



Shore-ice breccias. — In arctic regions shore ice often receives 

 a load of angular waste, and, drifting along the coast or out to sea, 

 desposits it as breccia amid the sediments of the ocean bed. 



The ice-foot, described by Feilden and De Ranee 3 as having its 

 origin chiefly in snows drifted into water offshore, receives the 

 waste of the talus slopes at whose base it lies. Ice floes along shore 

 also obtain a load of similar debris tobogganing out from talus 

 slopes and falling upon the floes from sea-cliffs. Shore ice may 

 also carry rounded beach pebbles frozen to its base and glaciated 

 pebbles shaped by the grinding of ice pans on shelving shores in 

 storms and under the action of the tide. 



The Quebec group of the Ordovician of Canada contains breccias 

 explained by Sir J. William Dawson as early as 1833 as due to shore 



'J. E. Marr, "Notes on a Conglomerate near Melmerby," Quar. Jour. Geol. 

 Soc. London, LV (1899), 11-13. 



2 E.g., the Wapsipinicon breccias of Iowa, W. H. Norton, Iowa Geol. Surv., IX, 

 447-48. 



3 H. W. Feilden and C. E. de Ranee, "Geology of the Arctic Coasts," Quar. 

 Jour. Geol. Soc. London, XXXIV (1878), 563-66. 



