1 66 STUDIES FOR STUDENTS 



the landslide mass, after reaching the foot of the mountain, poured 

 down the level valley floor for nearly a mile, covering it over its 

 whole width to a depth of more than 30 feet. 1 The section of a 

 rock-fall breccia taken normal to the cliff therefore resembles that 

 of the rock-glacier. No marked difference in the size of fragments 

 in different parts of the area covered has been recorded, so far as 

 the writer is aware. In the breccia formed by the landslide at 

 Frank, Canada, in 1903, described by McConnell and Brock, 2 

 large rocks are said to be common everywhere. 



In rock-fall breccias, fragments are of local derivation and of a 

 limited number of kinds of rock. No zonal arrangement is possible 

 in a single slide; but where repeated falls occur, the earlier bringing 

 down material from higher horizons on the mountain and the later 

 falls material from lower terranes, zonal arrangements both vertical 

 and horizontal may result. Thus Howe 3 explains the zone of 

 rhyolite which forms the outer rim of the Pierson Basin rock- 

 stream in Colorado, while the center of the mass consists of frag- 

 ments of andesite of a lower volcanic series. 



In rock-slides the movement is gradual and repeated. The 

 displaced mass, therefore, is not so completely broken up as in 

 the rock-fall, nor does it come to cover so large an area, except 

 under special conditions. No type of breccia contains such large 

 blocks as this. While the largest blocks of the Frank rock-fall 

 measured 40 feet, blocks 300 feet in diameter occur in the Rico 

 rock-slides, and in the profounder displacements of the adja- 

 cent telluride quadrangle one block of more than two miles in 

 length has been described. 4 On the north side of the Grande 

 Ronde River, Idaho, the Columbian lavas supply slidden blocks 

 half a mile in length. 5 The breccia of rock-slides is characterized 

 by the confused relations of the slipped blocks, their varying dip 



1 Sir W. M. Conway, The Alps from End to End (Westminster, 1895), 3d ed., 

 p. 246. 



2 R. G. McConnell and R. W. Brock, Ann. Rept. Dept. of the Interior, Canada, 

 1903, Part VIII. 



3 Ernest Howe, "Landslides in the San Juan Mountains, Colorado," U.S. 

 Geol. Surv., Prof. Paper 67, p. 34. 



4 Whitman Cross, U.S. Geol. Surv., 21st Ann. Rept., Part II, p. 146. 

 5 1. C. Russell, U.S. Geol. Surv., Water Supply Paper 53, p. 76. 



