The route lay on the eastern side of the archipelago, a distance of 1,184 miles. Leaving Oahu the depth increased quite rapidly until it reached 2,50o fathoms, at a distance due north of thirty miles. The line led along a plain from 2,50o to 3,00o fathoms in depth, save that there were two or three outlying peaks rising half-way or more to the surface. The average temperature of the surface water was 73.2° Fah. ; of the bottom about 35°.
The pelagic deposits were chiefly Red clay, Volcanic mud and Globigerina ooze. The first is the most extensive, being a smooth, sticky mud, from light yellowish brown to dark chocolate in color, and composed of clay, calcareous and siliceous organisms, mineral fragments of volcanic origin, and various products of local chemical formation, as nodules of manganese peroxide, crystals of phillips ite and particles of palagonite. The teeth of sharks and fish were not found in this section. The least depth at which the red clay was found was 2,010 fathoms. This material is supposed to have been derived largely from the pumice blown out from volcanoes and carried over all the oceans by currents. When thoroughly soaked it sinks and changes its color.
The volcanic mud consists of pumice, glass, ashes and the debris of volcanic rocks, more or less mixed with organisms at great depths. It has been derived from the volcanic masses of the several islands adjacent, and thus passes into the terrigenous deposits. The most abundant constituent is the glass, occurring as threads, masses from which the fibres were drawn out and angular transparent fragments. Red palagonite is more common
in this than in any other pelagic deposit.
The Globigerina ooze contains over 3o per cent. of calcium carbonate in the form of minute shells of foraminifera, of which the most common is that from which the name is given. The animals swarm in the waters above 2,200 fathoms and the dead shells accumulate at the bottom of the sea. Some idea of their appearance may be learned by inspecting Plate IA, showing the cast shells considerably magnified. When alive the surface of the sphere is covered by numerous spines suggestive of chestnut burrs, except that they are of very uneven lengths. When this ooze is brought to the surface and is solidified, it becomes chalk.
No one can satisfactorily estimate the thickness of these pelagic deposits. They must have been accumulating for several geo logical periods. Because of the presence of large amounts of calcium carbonate in the ocean of organic origin, the water has dissolved as much of that mineral as it will carry and also abounds in carbonic acid, which assists in dissolving other substances than calcium.
Certain other deposits common in other parts of the ocean—as the Diatom and Radiolarian oozes and the Blue and Green muds— are wanting in the materials brought up from the bottom along the line between Honolulu and Midway.