214: THREE CRUISES OF THE '" BLAKE." 



submarine tertiary formation which extends for several hundred 

 miles along the outer banks, from Cape Cod to George's Bank 

 and the Grand Banks off New^foundland, constituting perhaps 

 the solid foimdations of the banks themselves. 



The bottom of the Gulf Stream slope, from 70 to 300 fath- 

 oms, and a belt varying from sixty to a hundred and twenty 

 miles, is composed in great part of siliceous sand, with grains 

 of feldspar, hornblende, mica, glauconite, etc., fragments of 

 sponge spicules, diatoms, and a very large percentage of calcare- 

 ous orofanisms. Arenaceous foraminifera are often dredg^ed in 

 considerable quantities (from five to eighteen per cent). The 

 percentage of argillaceous matter increases rapidly with the 

 depth. At a depth of about 4:00 fathoms, it is not more than 

 ten per cent ; in 1,300 fathoms, nearly thirty-nine. The ab- 

 sence of argillaceous matter in the inner portion of the conti- 

 nental shelf (60 to 150 fathoms) is very marked. 



Professor Verrill has called attention to the floating beach 

 sand met with at a distance from shore, the fine argillaceous 

 sediment being carried out to sea to sink in greater depths near 

 the foot of the continental slope, or to mix with the globigerina 

 ooze in the track of the Gulf Stream or in the deeper waters 

 of the Atlantic. 



The conditions under which the shells of mollusks and the 

 hard parts of invertebrates may become fossil depend greatly 

 upon the habits of many of the deep-sea denizens. Some of 

 the deep-sea fishes, judging from the contents of their stomachs, 

 must dig up the muddy bottoms in search of their food. Others 

 again, like many of our shore fishes, swallow shells, which are 

 disgorged subsequently unaffected by digestion. Starfishes de- 

 stroy large numbers of mollusks, boring sponges and carnivo- 

 rous gasteropods many more, and the bottom of districts where 

 an abundant fauna exists must be covered with fragments of 

 remains of invertebrates in all possible stages of conservation, 

 ready to be imbedded in the muds, clays, or limestones forming 

 on the floor of the ocean. Holothurians and sea-urchins work 

 over the infusorial animals and smaller invertebrates living on 

 the bottom, by throwing them out again after swallowing them 

 for the sake of the organic matter they contain. Under the 



