352 
THE VOYAGE OF H.M.S. CHALLENGER. 
The nucleus of one nodule broke down into a floury material, which under the micro- 
scope seemed to be composed of a large number of prismatic crystals belonging to the 
monocliuic system. Besides the larger nodules to which we have referred, there was a 
considerable number of smaller ones varying from 0’5 to 2 cm. in diameter, almost all 
formed round minute fragments of pumice. Frequently numbers of these were cemented 
together by the manganese, and appeared to l)e in the process of formation into larger 
nodules. 
Station 252, 2740 flithoms. — The trawl brought up many hundreds of manganese 
nodules along with some rounded fragments of pumice ; there was no clay mixed with 
these nodules, having apparently been all washed away as the trawl was hauled up 
through the water. The largest nodules were about the size of cricket balls ; they were 
more or less round or ellipsoidal, and when rolled on the deck they looked like a pile of 
dirty potatoes. PI. III. fig. 5 represents (natural size and in section) the prevailing 
form, size, and structure of the nodules from this station. Three zones may be distin- 
guished in the figure : — (a) The elongated yellowish white centre or nucleus penetrated 
by dendrites of manganese ; it is hard and compact, and rather sharply separated from 
the dark layers which surround it. It may be observed that the elongated form of the 
nucleus appears to be the cause of the ellipsoidal form of the nodule, the nearly spherical 
nodules having a round nucleus, (b) The zone of manganese immediately surrounding 
the nucleus has a thickness of about 1 cm., and in it no concentric arrangement can be 
observed. This intermediate zone is generally terminated externally by a band of more 
compact manganese, sejDarating it in a manner from the more external layers, and appears, 
for many reasons, to have formed part of the original nucleus, which may possibly 
have been a fragment of pumice. There is almost always an interruption of continuity 
between the intermediate and outer zones, accompanied by a layer of light brown clay or 
mud. (c) In the outer zone there is a distinct concentric structure, determined by small 
alternate layers of manganese and clayey matter ; these layers have each a thickness of 
about 1 mm., and the depth of the whole zone is about 7 mm. The manganese in tliis 
zone is purer than in the others, and on a polished surface it has a semi-metallic lustre. 
PI. IX. fig. 7 represents a section of one of the round nodules. The manganese has here 
been removed by placing the face of the section in strong cold hydrochloric acid ; in this 
way a clayey skeleton is obtained showing distinctly the structure of the nodule. The 
three zones indicated al)ove may again be observed ; the nucleus, however, is small, 
having a diameter of only 2 mm. This is surrounded by an area showing no concentri* 
arrangement, then follows the outer zone with concentric layers. Fig. 7a represents a 
portion of the outer zone (c) magnified 25 diameters. The manganese has been removed 
and the empty spaces indicate the positions occupied by the manganese, which had a 
dendritic arrangement throughout the earthy or clayey matter. This clayey skeleton is 
fine grained, and is with difficulty held together. It may be remarked that the outer 
