REPORT ON THE DEEP-SEA DEPOSITS. 
853 
layers of the external zone (c), with a distinct concentric arrangement, have a very con- 
stant thickness of about 7 mm. for the majority of the nodules from this station, while 
the inner zones are variable in thickness. On the fracture of these nodules by a blow, 
they separated into large concentric scales. PI. III. fig. 5 represents (natural size) a 
nodule (7x7x5 cm.) broken to show the nucleus, which in this case is a large Car- 
charodon tooth, about 4 cm. in its greatest length ; the tooth is surrounded by concentric 
layers of manganese 1‘5 cm. in depth, and the whole nodule has roughly the form of the 
tooth. The tooth is black and shining, and is thoroughly impregnated with manganese ; 
the vaso-dentine has entirely disappeared from the centre, the hard dentine of the outer 
surface alone remaining. There were three or four other nodules with sharks’ teeth 
{Oxyrhina and Lamna) occupying the centres. PI. IV. fig. 1 represents the external 
form and aspect of a typical nodule from this station. The mammillae vary much in 
size, and are applied against and pass into each other without any very marked outlines ; 
each mammilla corresponds to a concretionary centre, and, when cut into, these parasitic 
concretions are found to be pieces of more or less altered pumice or small sharks’ teeth. 
Among the nodules were one or two that appear to have been broken while yet at 
the bottom of the ocean, and these fragments have subsequently formed the nuclei of 
other nodules. In some eases small fragments of palagonite are found in the centres of 
the nodules. The most frequent nucleus, however, is a hard white or yellowish substance, 
which, when examined in thin slices, is slightly transparent, but does not show any 
special structure to indicate its origin. In the fundamental mass little prismatic bodies 
are seen, but they have no characters which permit them to be referred to any mineral 
species. The fundamental mass appears to be composed of extremely fine grains, and 
sometimes there may be observed among these opaque points of manganese or fragments 
of sharks’ teeth ; between crossed nicols the mass behaves like an isotropic body, only 
some grains show, sporadically, birefrangence. When these nodules are broken down, 
crystals of hornblende, felspar, and magnetite may be extracted from the mass, yet it is 
extremely rare to observe these minerals in the microscopic sections. Among the mag- 
netic particles are also metallic spherules of cosmic origin. Between twenty and thirty 
pieces of pumice were among the manganese nodules ; these were either highly altered 
at the surface or surrounded with a coating of manganese 0'5 cm. in thickness. 
It may be noticed that an analysis of the clay brought up in the sounding tube 
yielded only traces of manganese ; the trawl, however, here yielded one of the largest 
hauls of manganese nodules taken during the cruise. It would appear as if the trawl had 
been dragged over a considerable surface of the deposit, the nodules being retained by 
the net while the clay in which they were imbedded was washed away. If this be the 
correct interpretation it is quite possible that the nodules are but sparsely scattered 
throughout the deposit, and that they had segregated nearly all the manganese from the 
clay. The quantity of manganese in the clay in which the nodules were imbedded in 
(deep-sea deposits chall. exp. — 1891.) 45 
