1006 
pieces. The nodules have often a white or gray nucleus free 
from manganese in their centre, around which on all sides the 
manganese has been precipitated in concentric, porous layers *). The 
white nucleus is sharply contrasted with the dark envelope (Fig. 3 
€ 
in the text and PI. II fig. 3). 
4 
Fig. 3. Manganese nodules with a white nucleus of chert containing 
radiolaria for the greater part altered into amorphous silica. 
In some of the specimens part of the nuclear mass is of a greenish 
colour and dimly transparent. These parts can easily be recognized 
as chert with a strong pocket lens. 
The nuclei are always brittle and more or less friable. In one 
case 1 succeeded in having a thin section made through an entire 
nodule, nucleus and envelope, without interfering with the structure. 
This slide is reproduced in PI. [ fig. I. 
On microscopic examination this nucleus appeared to consist of 
radiolarite, being converted for the greater part into white, amorphous 
silica. In it the radiolaria are packed close together, their casts being 
filled up with a crystalline mosaic of quartz. The concentric arrange- 
ment and the porous character of the manganese envelope round this 
nucleus are easily recognizable in this figure. In some other slides 
'! Very rarely a white substance free from manganese, quite similar to 
that of the nuclei, was found outside the centre of a nodule, between two 
layers of the manganese envelope. 
