510 



GEOLOGY. 



or in secondary deposits, many of which are comparatively recent. 

 Of the less superficial finds, many have been shown to be cases of second- 

 ary burial by natural means. The usual modes followed by streams 



mmmimw/. 



Fig. 572. — A gravel bluff formed by the under-cutting of the adjacent river. 



(After Holmes.) 



in cutting down their channels in valley gravels are peculiarly well 

 suited to bury superficial material to very considerable depths, for 

 in their meanderings they cut into the bordering terraces or uplands 



"mmmmmz, 



Fig. 573. — The same at an early stage of talus formation. 



at intervals and develop steep bluffs. When the meanders shift, 

 as they are sure to do, the bluffs inevitably grade down to a slope by 

 the falling, or sliding, or washing of the top to the bottom, as illus- 



Fig. 574. — The same at a late stage of gradation, when the slope has become 



nearly stable. 



trated in Figs. 572-574. What was in the top portion naturally 

 becomes part of the base of the talus, and is deeply buried. Similar 

 secondary burials take place in various kinds of loose material, includ- 



