THE DEPTH AND MARINE DEPOSITS OF THE PACIFIC OCEAN. 147 
being covered with numberless closely set mammille, giving it the appear- 
ance of shagreen or coarse leather. On account of the presence of these 
mammille — 1 mm. in diameter, as a rule — the surface is dull, not shining. 
The color is brown, never black. The external layer, from 1 to 2 mm. in 
thickness, peels off easily. In some cases the layer coming next to it 
shows shagreen structure; in other cases it does not, being smooth. This 
external layer is generally the only one that can be removed; the rest of 
the nodule is very compact, and so hard as to be scratched with a knife 
only with difficulty. When whole and with no fracture previously exist- 
ing, the nodules cannot be broken in the hand, even when great strength 
is exerted ; the specific gravity is very high. 
All the nodules that were cut in order to study their structure were 
found to be formed round a nucleus, which is relatively large and in- 
variably composed of a palagonitic tuff. The palagonitic substance form- 
ing the nucleus varies from a creamy white to a reddish color, the white 
portion being relatively soft and incoherent, whilst the red portion is so 
hard as to be cut with a knife only with difficulty. This tuff is most inter- 
esting, inasmuch as one can follow under the microscope its mode of forma-~ 
tion: it is undoubtedly due to the decomposition of a volcanic glass, a few 
particles of which can still be recognized, as they have only partly lost their 
transparence, and can be seen to pass into opaque palagonite. A few scarce 
particles of partially decomposed olivine accompany the palagonite, together 
with rare particles of felspar. The presence of augite cannot be made 
absolutely certain, but it probably occurs together with the other minerals. 
These particles are only a few hundredths of a millimeter in diameter, but 
in one case the centre of a nucleus is occupied by a flake of volcanic glass 
about 2 mm. in diameter. 
The nucleus occupies a large portion of the nodule, its diameter varying 
from one-third to one-half of that of the whole nodule. 
The manganese-iron oxides portion of the nodule does not pass 
gradually into the palagonitic nucleus, but envelops it without pene- 
trating into it, so that a nucleus can be decorticated simply by fracturing 
the nodules and removing the manganese layer. The latter is very com- 
pact and homogeneous, so that indications of a concentric arrangement of 
the layers can hardly be made out, with the exception of the external, 
shagreen-like one. One of the nodules shows a peculiar structure; the 
nucleus is surrounded by a layer of oxides a centimetre in thickness, 
