62 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 27 



of the shelves and they contain shallow-water foraminifera and remains 

 of other animals and plants, including bits of wood from land. Within 

 the canyons the sands form narrow bands traversing the canyon axes 

 between the steep walls covered by green mud. Movement of this mud 

 downslope to the intermittently moving axial sand produces the observed 

 bedded character of the sediment on the floors of the canyons. The sands 

 in canyons near the mainland contain lower percentages of calcium car- 

 bonate than do the muds, in agreement with the low content of calcium 

 carbonate in sands atop the mainland shelves as compared with that of 

 muds on the basin slopes and floors. In contrast, the sands in offshore 

 canyons have more calcium carbonate than the muds, again in response 

 to the shelly nature of sands of island shelves and bank tops. 



Third, and least important, are small quantities of sediment from the 

 outer parts of the shelves which are moved into the canyons, probably 

 by occasional storm waves. Their presence is attested by occasional grains 

 of glauconite and phosphorite, authigenic sediments which are most com- 

 mon on bank tops and on the outer parts of shelves. 



As shown by Menard (1955) and by Emery (1960a), the quantities 

 of sediment in sub-sea fans and aprons far exceed the volume of rock 

 which has been removed during erosion of the canyons. Since the fans 

 consist mostly of sand, it is evident that the canyons act as conduits for 

 movement of sand from near shore to deep water. As pointed out by 

 others, this movement may act as a sort of giant chain saw cutting down- 

 ward into the bedrock floors of the canyons. Deepening of the axes 

 steepens the side walls and allows more sliding of muds from the canyon 

 walls, possibly leading to lateral enlargement of the canyons. Future 

 work from manned or televised deep-diving vehicles should go far toward 

 investigating this interesting geological agent of erosion. 



Downcutting of canyon axes by moving axial sands should clear away 

 a strip through the blanketing muds or prevent the muds from being 

 deposited. Any aquifer which has been exposed through erosion by the 

 same sand or by other possible canyon-forming agents is thereby exposed 

 to the sea water. If the internal water pressure is greater than hydro- 

 static pressure of sea water at the outcrop, fresh water should leak to 

 the sea. If the reverse is true, owing to over-pumping or perhaps to 

 natural causes, sea-water intrusion should occur. Because of widespread 

 over-pumping in the intensely cultivated and highly populated coastal 

 areas of southern California, sea-water intrusion is well known. It is 

 generally made manifest by increasing salinity of water wells (Emery, 

 1960a). Deeper aquifers, largely untapped by water wells, may be 

 expected to behave differently than the over-pumped shallow ones. 



