STUDIES FOR STUDENTS 165 



by torrential streams upon detrital slopes. It is concluded that 

 heavy trains of debris saturated by meltings of thick snows have 

 moved en masse down the steep slopes and out over the lower 

 grounds below. The junior author reaffirmed this theory in 1881 

 and compared the breccia directly with the talus glaciers of the 

 Rocky Mountains. 1 



A matter perhaps less satisfactorily explained is the frequent 

 brecciated aspect of the undisplaced limestone of the Rock of 

 Gibraltar. In the words of the authors cited, "In many places 

 the Rock looks as if it had been smashed up in situ, the broken 

 fragments having been subsequently consolidated by infiltration." 

 The authors prove that the shattering is not due to faulting, and 

 attribute it to frost. The fractures recall, however, those of the 

 country rock of the landslide area of the Rico Mountains, referred 

 by Cross 2 to prehistoric earthquakes of exceptional violence. 

 Indeed, the features of the Gibraltar breccias as described by Ram- 

 say and Geikie are not inconsistent with an origin in landslides, 

 which also move out over gentle slopes and carry large fragments 

 to the outer limits. 



Landslide breccia. — Breccias of this class owe fragmentation in 

 large measure, and assemblage wholly, to the force of gravity. In 

 rock-falls the movement is sudden and violent, and the rock is 

 shattered to pieces by successive impacts. Rock-fall breccia is a 

 chaotic mixture of blocks, large and small, set at all angles and 

 sharply angular. In the few seconds of the tremendous downrush 

 of perhaps millions of tons of rock, the fracture of the masses to 

 smaller and still smaller fragments and their crush to powder go on 

 so rapidly that comparatively little opportunity seems to be given 

 to the rounding of edges by attrition. The matrix consists of 

 chinkstone and pulverized material and may embrace a contribu- 

 tion of soils and subsoils swept up by the rock-torrent. 



Large rock-falls on steep slopes obtain sufficient momentum to 

 carry them a considerable distance over gentle and even reversed 

 gradients at the mountain's base. In the Elm rock-fall of 188 1 



1 James Geikie, Prehistoric Europe (London, 1881), p. 219. 



2 Whitman Cross, "Geology of the Rico Mountains, Colorado," U.S. Geol. Surv., 

 21st Ann. Rept., Part II, p. 149. 



