274 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 Newfoundland, constituting perhaps 
the solid foundations 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 organisms. Arenaceous foraminifera are often dredged in 
considerable quantities (from five to eighteen per cent). The 
percentage of argillaceous matter increases rapidly with the 
depth. At a depth of about 400 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 
