REPORT ON THE DEEP-SEA DEPOSITS. 
367 
— -like pumice and glassy lapilli, almost always profoundly altered — are perhaps the most 
frequent nuclei, then follow teeth of sharks and other fish, otoliths, bones of Cetaceans, 
siliceous and calcareous Sponges, and even agglomerations of the deposits in which casts 
of Foraminifera can be recognised. 
Not only the external form and the presence of nuclei, but also the internal structure, 
indicate the concretionary nature of the nodules ; the sections, in fact, show that the 
nodules are built up of successive concentric zones. The inner zones follow closely the 
form of the nucleus, while those towards the exterior are more regular and have more 
ample curves. Some zones are darker than others, and in these the manganese is more 
abundant than in the intervening ones, which have a large admixture of earthy and 
clayey materials. The zones vary in thickness in different specimens ; sometimes they 
are thinnest in the central, and sometimes in the outer, layers. This zonary structure is 
well exhibited when the nodules are demanganesed ; the clayey and earthy skeletons that 
remain after this treatment resemble strikingly all the varieties of urinary calculi. The 
empty spaces in these skeletons show the positions occupied by the eliminated manganese 
in the nodules, and it may be seen that the dendrites had passed across the earthy and 
clayey zones. 
The concretionary arrangement of the nodules is likewise clearly exhibited by the 
facility with which the successive zones may be separated into concentric shells or scales 
following the earthy layers. In some of the more compact and purer nodules, and in 
spaces free from foreign substances, a distinct fibro-radiate disposition may also be 
observed, recalling the structure of pyrolusite, and there is nearly always a tendency to a 
fracture following the radii of the nodule. Some of the nodules, indeed, have broken up 
in this fashion while still at the bottom of the sea, and the separate fragments or wedges 
of the original nodule have become the centres or nuclei around which new concentric 
layers have been deposited. 
Microscopic Characters . — The microscopic characters of the manganese concreted in 
the nodules do not present any peculiarities to allow of a specific determination of the 
mineral. Like all the oxides of manganese, it appears, in the thin slices of the nodules, 
as absolutely opaque ; a black mass sometimes with a brownish tint. There is no trace 
of internal structure nor of crystalline form, if we except some small patches in a few of 
the denser nodules, whose crypto-crystalline appearance has been compared to pyrolusite. 
When mixed with the elayey matters of the deposits the manganese is often seen as 
minute roundish grains with a black opaque centre and a brownish coloured border. But 
generally the red-brown or chocolate pigment of the deposit is indefinite, and the oxides 
of iron and manganese occur with very vague contours. In the nodules the manganese 
appears to be amorphous, but as we have said it assumes a dendritic arrangement which 
can be well seen under the microscope. All the details of this structure, and the form 
of the manganese in the nodules, are represented by the figures on Pis. XXVIII. and 
