68o PRINCIPLES OF STRATIGRAPHY 



Interruptions of Marine Sedimentation. 



It is generally assumed that deposition in the deep sea is rela- 

 tively constant and not subject to interruptions of any but acci- 

 dental character. Under this latter may be classed the appearance 

 of submarine volcanoes, which will act in a twofold manner by dis- 

 turbing the waters of the bottom of the sea and creating currents 

 which will stir up and sweep away sediment previously accumu- 

 lated, and, second, by forming a new series of deposits, as well as 

 creating new slo])cs and regions of deposition. Seismic disturbances 

 likewise cause interruptions of sedimentation and rearrangement of 

 sediments. 



In the lesser depths of the ocean, however, the currents of the 

 surface will to a certain extent also afifect the bottom sediment. 

 This is partly due to their ability to sweep away loose material 

 even in considerable depths and partly to their influence in pre- 

 venting sedimentation. As already noted in Chapter V in narrow 

 passages, the currents may be effective at considerable depths. 

 Agassiz states "that the bottom of the Gulf Stream along the Blake 

 plateau is swept clean of slime and ooze, and is nearly barren of 

 animal Hfe." (i '-^39.) This effect is felt to a depth of 1,281 me- 

 ters. According to Verrill, the floor of the ocean beneath the Gulf 

 Stream in depths of 150 to 600 meters, and at a distance of 100 to 

 200 km. from land, is covered with fine sand, mostly quartz, some 

 feldspar, mica and magnetite, fragments of shells, etc., coral and 

 rhizopods. Fine mud is absent and is even scarce in depths of 

 1,000 meters, having apparently been carried away by the Gulf 

 Stream. A current of more than 7 mm. per second at the bottom 

 can stir up and move shell particles of 0.12 mm. or less in diameter 

 and be quite effective in transporting mud particles. Currents of 

 3 mm. per second can carry along Globigerina ooze. Measure- 

 ments on the Gulf Stream have shown a velocity of about 31 mm. 

 per second at a depth of 910 m. west of the Bermudas. In 1,100 

 m. depth, however, no measurable velocity was shown. 



As already noted (Chapter V), the passages between the several 

 islands of the Canary group have their bottoms kept clean by the 

 tidal currents rushing through theiti. These currents are effective 

 to a depth of 2,000 meters. On the submarine banks in the neigh- 

 borhood of these islands the denuding eft'ect of the tidal currents is 

 felt to considerable depths. On the Seine bank only hard rock bot- 

 tom was found in depths of less than 200 meters. In greater depths, 

 down to 964 meters, only coarse sand was found. Only in the 



