EEPORT ON THE DEEP-SEA DEPOSITS. 
345 
treated is represented in fig. 36.^ This examination shows likewise that the yellowish 
white matter extends into and across the dark bands. It is also to be noted, as seen in 
fig. 3a, that these alternate bands are not continued quite to the edge of the section, but 
are surrounded by a black layer in which no alternation of light and dark bands is at once 
visible, although when a surface of the section is demanganesed, alternating bands may 
be distinguished, the lines being very fine. This external black layer is much more 
compact than the internal portions of the nodule, and follows perfectly the contours of 
the triangular wedge, covering the whole of the periphery. The internal parts are 
much more friable and porous than the external layer, and the separation between them 
is very well marked and rather sharp. Sometimes there is an interruption of continuity 
between the internal concentric alternating layers, which causes this variety to break into 
coatings and peel like an onion more easily than the spherical variety. There is no 
central nucleus in this pyramidal variety, unless the whole interior be regarded as a 
nucleus surrounded by the layers forming the black border. There were some twenty 
or tlfirty nodules of this variety, but large numbers, although presenting certain analogies 
with these typical forms, are much more irregular. 
Of the nodules which we would designate as grape-shaped, it is impossible to give a 
morphological description. This arises from the fact that the mammillae are superposed 
the one on the other, so as to recall a bunch of grapes, or they may present all the 
irregularities of certain volcanic scoriae. The majority of these irregular forms, however, 
have internal alternating bands, more or less resembling those shown in fig. 3a. The 
peculiarities of this pyramidal and irregular variety of nodules might be explained by 
supposing the central parts with the alternating bands to have once formed parts of a 
larger nodule, which had been broken up along the radii, and these broken fragments 
to have been subsequently surrounded by the deposition of the more compact external 
layers. 
There were about fifteen nodules belonging to the second, spherical or ellipsoidal, 
variety, resembling in form the nodule figured on PI. IV. fig. 8 from Station 276. They 
have a diameter of 1 to 5 cm., are much less mammillated than the irregular varieties, 
and consequently preserve their spherical form more or less perfectly. They have a fine 
concentric structure, like that represented in PI. IX. fig. 7, showing a section of a nodule 
from Station 252. The zones surround a central nucleus of volcanic glass, palagonite, 
shark’s tooth, or bone ; two palagonite nuclei are shown in PI. XVI. fig. 2 and PI. XVII. 
fig. 3. Sometimes, however, there is no apparent nucleus. These nodules are more 
compact, heavier, and break less easily than the preceding variety. Their fractures are, 
however, very well defined, and always follow the rays and concentric layers. They 
take a beautiful metallic polish, and on the polished surface the fine concentric arrange- 
1 The dendritic arrangement of the manganese is well seen in the thin sections of the nodules under the micro- 
scope, as shown in PI. XXVIII. figs. 1, 2, 4, 5. 
(peep-sea deposits chall. exp. — 1891 .) 
44 
