128 THE FLOOR OF THE OCEAN 



eastern United States were excavated in post-Miocene time, as 

 proved by Dr. Stetson's discovery of fossiliferous Miocene sedi- 

 ment outcropping on the canyon walls. 7 Since these canyons 

 were dug, their floors have been raised a few feet or inches by 

 the recent deposition of fine muds, whose fossils denote a de- 

 cided warming of the Atlantic during this limited sedimen- 

 tation. Beneath that muddy layer are sediments containing 

 shells of cold-water animals, which are reasonably believed to 

 have lived during the last (so-called Wisconsin) stage of Pleis- 

 tocene glaciation on the lands. This assumption implies that 

 some of the canyoning of the continental slopes is to be referred 

 to pre- Wisconsin time, though not necessarily to pre-Glacial 

 time. 



The likeness of the furrowed submarine slopes to rain- 

 gullied slopes on the lands early prompted the idea that sub- 

 aerial streams excavated also the submarine canyon. This 

 hypothesis was first elaborated by James D. Dana, following 

 A. Lindenkohl. It was argued that the continental terraces 

 were uplifted, so as to be for a short time 5000 to 10,000 feet 

 above sealevel; that during this high stand the canyons were 

 cut by rain-fed rivers; and that, after the deep trenching, each 

 continental slope sank bodily, with drowning of the new can- 

 yons. A few students of the problem still prefer that hypothesis, 

 and also extend it to cover the intense furrowing of the conti- 

 nental slopes — a feature not known to Dana and Lindenkohl. 



Several weighty objections of the conception led Professor 

 Shepard, long after, to assume subaerial excavation of canyon 

 and furrow, not because of temporary uplift of the continental 

 terraces but because of temporary, world-wide sinking of sea- 

 level, the continents suffering no distortion. To describe such 

 a general withdrawal of the ocean we have the technical word, 

 eustatic. If the supposed eustatic shift were great enough and 

 long enough continued, canyon and furrow could be the- 



