Johnston — Imbricated Structure in River-gravels. 389 



is, therefore, of economic importance and its character 

 should be clearly understood. 



Becker also pointed out that "On beaches pebbles are 

 sometimes imbricated for a few feet in one or another 

 direction and sometimes lie nearly flat. The constant 

 reversal of the currents due to breaking and retreating 

 waves prevents any extensive methodical arrangement, 

 and this fact is of assistance in discriminating marine 



Fig. 2. 



Fig. 2. — Vertical section of conglomerate, on Burrard inlet, near Van- 

 couver, British Columbia, showing imbricated structure. The direction of 

 the current was from left to right. 



gravels from river deposits." 5 This criterion for dis- 

 tinguishing marine from non-marine deposits does not 

 appear to have received much consideration by geolo- 

 gists, probably because of its limited application, but it 

 may be occasionally used, as pointed out below, and is of 

 importance because of the fewness of such criteria. 



Imbricated structure is usually best shown in the very 

 coarse gravel and shingle deposits of river beds, but is 



5 Opus cit. p. 54. 



