INTERRUPTIONS OF MARINE SEDIMENTATION 68i 



depths exceeding 1,500 and 2,000 meters is an accumulation of the 

 fine (Jlobigerina ooze possible. 



In the passes between the islands of the Indian Archipelago, the 

 Siboga expedition found mostly "hard bottom," the sounding in- 

 struments bringing up only broken-off rock fragments, or impres- 

 sions of a rock bottom. In exceptional cases coarse sand was 

 found. Since these depths extend to 1,500 meters, it is probable that 

 we have here a prevention of sedimentation by the removal of the 

 particles before they reach the bottom, rather than any eflfect of 

 eroding work of the current at such depth. (Weber-ioi : i8l.^ 

 Compensatory currents, such as those formed in the Roman Med- 

 iterranean by greater evaporation, also affect sedimentation. In the 

 case mentioned the Gibraltar current flows in at the surface, while a 

 deeper current passes out beneath, affecting the lower 30 meters of 

 the water in the pass. Currents due to melting ice also affect sedi- 

 mentation. The diatoms of the Antarctic region settle only north 

 of the region of drift ice (Philippi). In addition to the oceanic 

 and the tidal currents, the upward currents, which bring the colder 

 water of the depths to the surface, are effective agents in modifying 

 deposition. All such currents are further effective in determining 

 the facies of the sediment, and abrupt changes from one to another 

 type of sediment may often be due to them. 



It thus appears that sedimentation on the sea bottom is unequal 

 in extent and that the surface of more elevated ridges, or areas 

 subject to current scour, may remain free from sediment, while this 

 is accumulating all around. Cases of interruption of the continuity 

 of sediments referable to such causes are not unknown from the 

 older rocks. A good example is found in the Upper Siluric beds 

 of the Helderberg Mountains in eastern New York. Near the city 

 of Kingston a submarine ridge of folded and eroded sandstones of 

 Mid-Ordovicic age projected above the general bottom of the late 

 Siluric sea, and was kept free from the deposits accumulating all 

 around it. Thus the Rosendale waterlimes and the Wilbur lime- 

 stone are wanting over the Kingston ridge, but present all around 

 at a distance of a few miles. Only the upper part of the Cob- 

 bleskill limestone is deposited upon the ridge, which became buried 

 in the deposits accumulating around it, to which, however, the 

 ridge itself did not contribute any material. These deposits, how- 

 ever, accumulated in shallow water, as lime muds derived by the 

 erosion of a not too distant limestone shore. The shallow water is 

 indicated by the abundance of mud cracks at several levels. The ab- 

 sence of the Credneria beds and of the carinata-quader (sand- 

 stone) in the vicinity of Dresden (Plauenscher Grund) has also 



