156 THE FLOOR OF THE OCEAN 



that canyons are now being slowly filled with mud; (5) the 

 discovery of cold-water shells (foraminifera) in the clay that 

 underlies the new muddy layer of incipient filling; and (6) the 

 departure of the existing continental shell from a profile of 

 equilibrium. 



Incidentally our study of silty currents has indicated a 

 mechanism for the transport of shore sediment to water depths 

 of 2000 fathoms and as far as 100 to 200 miles from the land 

 where the sediment had been manufactured. 20 This long 

 travel of sand and mud has long been a puzzle for geologists. 



We have seen that the silty-current hypothesis is favored by 

 the weakness of the terrace material, by the steepness of the 

 continental slope, by the visible proof that shore waters are 

 muddied through agitation by storms and tides, by laboratory 

 tests, by the teaching of Nature in the Swiss lakes and in arti- 

 ficial reservoirs, and by the failure of all other hypotheses yet 

 offered to account for the submarine valleys. 



Perhaps a hypothesis with so many supporting advantages 

 can qualify as a theory. It cannot represent demonstration. In 

 the geological workshop, chips and shavings of uncertainty still 

 lie around. They show that the carving of a complete image 

 of truth about the submarine valleys must be regarded as 

 unfinished business. 



And a word about the workshop itself: it is that of Louis 

 Agassiz, enlarged by his son Alexander, and by his grandchil- 

 dren. The great Swiss master came to Harvard's center of 

 scientific research soon after he had proved the former, whole- 

 sale glaciation of northern Europe and North America. 



The epoch-making discovery of Louis Agassiz has been con- 

 firmed by a small army of enthusiastic geologists who, in its 

 testing, have traversed the earth from pole to pole, across every 

 continent and across islands in every ocean. After much 

 patient labor they have mapped the glaciated tracts. Moreover, 



