Febbuabt 18, 1910] 



SCIENCE 



249 



which are nevertheless apparently less 

 complete than others in adjacent basins 

 and which seem therefore to have been 

 areas of non-deposition. Even if the hiatus 

 be real, and not merely supposititious, it 

 does not follow that non-deposition has 

 been a subaerial condition. Such anoma- 

 lies of non-deposition occur characteristic- 

 ally between strata laid down during 

 periods of wide-spread marine transgres- 

 sion when lands were low and covered with 

 residual or alluvial deposits. If any area 

 was raised higher during the interval, it 

 must have been correspondingly corraded. 

 And if the evidences of decay or corrosion 

 are wanting the postulate of a land area 

 corresponding to the region of non-deposi- 

 tion should be regarded with much doubt. 



Inconstancy of Marine Sedimentation. — 

 It is commonly assumed that sediment of 

 some sort necessarily accumulates over the 

 bottom of a marine basin and that this has 

 always been the case in epicontinental seas 

 of all ages. Consequently non-deposition 

 is not considered and special hypotheses of 

 uplift and subaerial erosion are devised to 

 account for the absence of strata which 

 might or should have been deposited. Tet 

 non-deposition and even the scouring of 

 bottoms so that hard rock is exposed are 

 conditions of modern sea bottoms where 

 they are swept by currents whose load is' 

 less than their efficiency. 



Verrill has described the coarse shifting 

 sands of the New England coast, which are 

 kept in such constant motion by tidal cur- 

 rents that no life finds lodgment on them. 

 The whole continental platform from Long 

 Island to Hatteras is so swept that sand 

 alone comes to rest, all finer sediment being 

 carried on to the zone of oceanic ooze. 



Agassiz found hard limestone bared of 

 any deposit except serpularia and similar 

 clinging organisms beneath the silt-laden 

 Gulf Stream, where it flows across the epi- 



continental platform, between Florida and 

 Cuba. Among existing seas and straits 

 this instance is one which, in the conditions 

 for marine scour, most nearly resembles the 

 epicontinental seas of past times. 



Between Scotland and the Faroe Islands 

 stretches the Faroe Island ridge, a wide 

 stony bar between the North Atlantic and 

 the Arctic basins. Its crest lies 300 fath- 

 oms below the surface of the ocean; yet it 

 is swept clean, while banks of ooze accumu- 

 late on the slopes north and south of it. 



The present distribution of lands and 

 oceans is unfavorable to marine scour and 

 favorable to deposition. Epicontinental 

 seas are confined to the margins of conti- 

 nental platforms, to which high lands 

 contribute abundant sediment, or they are 

 deeply embayed and shut off, as Hudson 

 Bay is. Non-deposition is therefore an ex- 

 ceptional condition. We may grant that it 

 has always been restricted to comparatively 

 shallow waters, in the path of a relatively 

 strong marine current. But the epicon- 

 tinental seas of the periods of great marine 

 transgressions (Cambrian, Ordovician, Si- 

 lurian, Devonian, Mississippian and Cre- 

 taceous of North Amex'ica for instance) 

 opened channels across the continent, 

 through which oceanic currents circulated 

 as the Gulf Stream flows from the Carib- 

 bean to the Atlantic. Low lands bordered 

 these seas and the deposits which accumu- 

 lated in the deeper basins consisted in 

 great part of fine calcareous ooze. Under 

 these conditions non-deposition and ma- 

 rine scour have been favored on shallows 

 along shores and in straits, and in any 

 such places a corresponding hiatus must 

 occur in the stratigraphie sequence. 



In paleogeographic study it is important, 

 therefore, to consider the principle that 

 marine waters may not only deposit sedi- 

 ment, but may also prevent deposition, or 

 even remove a deposit previously made. 



