8 



and the terrestrial conditions which led to the origin of these 

 great deposits?" Are deposits of this sort being laid down today? 

 What, if anything, have bacteria to do with the segregation of iron? 

 How does the concion association of iron with manganese in modern deep- 

 sea deposits bear on this problem of iron? How sound are the chemi- 

 cal reactions that have been proposed to account for the deposition 

 +C ^l f.^ °^ these minerals, and what conclusion must we draw, as to 

 the depths of the Paleozoic seas, from the fact that today it is only 

 m deep water that the bottom sediments contain a considerable per- 

 centage of either of these metals? 



Similar problems also arise in connection with the rarer aetals 

 the metamorphic limestones, silicious deposits, etc. With sillce 

 constantly contributed by the rivers to the sea, and with no back 

 loss either to the atmosphere, or to the land (except in regions of 

 elevation) it is obvious that the silica of the earth is now tenc^ing 

 to accumulate on the sea floor. The geologist is, therefore, as 

 deeply interested, as is the biologist, in the factors that C3,use such 

 accumulation of silica to take place most rapidly in cold water, and 

 at great depths, as signboards to the conditions under which accumu- 

 lations of silica occurred in the old seas. 



Analysis of the depths at which coarse sediments are now accumu- 

 lating, and of the role played in this connection by the scouring 

 action of waves, tides, and currents as a governing factor, is 

 instructive from the geologic point of view because the conglomerates 

 and breccies that were formed in the old seas have their equivalent 

 in the gravels and sands that are being deposited around the shores 

 of the oceans today. In like manner, a study of the blue muds around 

 the continental shoals and on the shelves is important because of the 

 probability that many of the shales laid down under the seas of old 

 were deposited in the same way. We think especially of the genesis 

 of the Paleozoic black shales of vast eztent, as to whose oripin there 

 are nearly as many theories as students. Modern deposits on the 

 submarine slopes of the oceanic islands, also bear on the sedimentary 

 ^ll^tL^L^''''^^^^'^^^ ^^ pointed on Page 3 . On the steep slopes 

 into the abyss, special watch should be kept for rock masses that 

 have broken away from the shelf and slipped down the slope. 



The regional distribution of the different types of oceanic 

 sediments, when we learn the correct interpretation, will throw lipht 



?lmt wh???i'^ °^ '"^^ permanence of the basins. Is'it safe to aasSSe 

 that where a basin is now floored with red clay (the most typical 

 abyssal sediment), it has continued deep for geologic ages pastrand 

 that ^ the presence of the teeth of sharks, and earbones tf whaleL of 

 species long since extinct that our dredges often bring up fron red 

 Te?tia?3 ?i^pf ^?^ 1^^"-* ;S* "r^'S^ sediment has sifted down, since 

 mlv tM J .iTh h ^""F *^-^^ ^^^P^^ ^^^^ ^^^ instruments scrape? Or 

 reLatellS tppf ^"""^ ^^^ ?'S? repeatedly covered by lime ooze and as 

 repeatedly freed from the latter by the solvent action of the water 

 with successive uplifts and sinkings of the sea floor? 



tiri.-n!! ^^'^''^^^ly ^"^i"}^' Of -fche terrigenous detritus around the cor- 

 tSr L! accumulating faster thando any of the oceanic oozes. How, 

 SfS'/^ ""^ ■^'' interpret the fact that glacial pebbles have often 

 been dredged, and over a wide range of depths? Is the depth ?o wl Ich 



