STUDIES FOR STUDENTS 173 



port should be looked for in disturbances in the bedding of the 

 inclosing strata. Laminae beneath the larger stones may be bent 

 down, and the succeeding laminae may show the influence of the 

 projecting blocks, thus proving that the fragments were dropped 

 through some depth of water. Confirmation has been found in the 

 position of fragments with the heavier ends downward. 



Raft breccias are endostratic and the fragments are sporadic. 

 There may also be an irregular distribution of them — in places 

 a huddle of fragments where the unloading of the raft was sudden. 

 Some blocks may be quite too large for wave and current carriage. 



Rafts capable of transporting the material of breccias are either 

 of ice or of vegetation. Ice rafts include both icebergs and shore 

 ice in the form of floes, or of the ice-foot. 



Iceberg breccia. — Since the iceberg is detached from the tide 

 glacier, iceberg breccia is composed of the material of the ground 

 moraine. In a larger* or smaller proportion the fragments prove 

 their derivation by their subangular form and striated faces. 

 A considerable lithologic variety is to be expected, since the parent 

 glaciers usually drain a large extent of country. Transport from 

 distant sources has long been looked upon as evidence of iceberg 

 carriage, since icebergs drift farther than other rafts. 



In weighing the evidence of iceberg breccia, and of glacier 

 breccia as well, it is often necessary to discriminate glaciation 

 of pebbles from slickensides by earth movements which affect 

 the mass of the formation. In favor of glaciation is the incrusta- 

 tion of planed or striated surfaces by marine organisms, such as 

 serpula or shells, since these surfaces must have been produced 

 before the deposit of the pebbles in the breccia beds. 1 



Striae may be considered "rutsch striae" produced after the 

 deposit of the breccia under the following conditions: (1) when they 

 occur on matrix as well as pebbles; (2) when they are found on 

 different planes below the surface of the pebbles; (3) when they 

 affect traceable planes or zones of shear; (4) when the striae of 

 different pebbles in the same plane run in the same direction and 



1 1. C. Russell, "Second Expedition to Mt. St. Elias," U.S. Geol. Surv., 13th Ann. 

 Rept., p. 25; W. J. Sollas and A. J. Jukes, "Included Fragments of the Cambridge 

 Upper Greensand," Quar. Jour. Geol. Soc. London, XXEK (1873), II_I 5- 



