Chap. XX.] 
DEEP-SEA DEPOSITS. 
403 
into the sea by rivers. My ideas as to the origin of the manganese 
Suniinaiy of origin of and its method of precipitation can be summed 
deep-sea nodules. ^p as follows :— 
1. The manganese, although probably partly derived from cosmic 
dust and volcanic debris, has been mostly precipitated from 
solution in the sea-water, the manganese salts having been 
originally brought into the sea by rivers. 
2. The manganese oxide, although possibly partly precipitated as a 
result of the action of the vital processes of organisms, both 
vegetable and animal, has been mainly precipitated by cal- 
ciimi carbonate aided by the obscure process of segregation 
from solution round a nucleus i. 
3. Where the sea-bottom consists largely of calcareous sediments, 
the precipitation may have been mainly brought about by the 
solution of some of this calcium carbonate with the deposition 
of an equivalent amount of manganese oxide owing to the 
presence of free oxygen. 
4. Where the sea-bottom consists of red clay, it does so because the 
depths are there so great that the tests of thin-shelled organ- 
isms are completely dissolved by the sea-water before they 
reach the bottom. The calcareous matter in being dissolved 
deposits an equivalent amount of manganese oxide, which 
descends to the bottom, and there acts as a nucleus for the 
segregative extraction of manganese from the waters at the 
sea-bottom. The deposition of manganese oxide by means of 
calcium carbonate associated with the red clays probably also 
occurs to a subordinate extent, for the shells of thick-shelled 
organisms may reach the bottom before being entirely dis- 
solved. 
There is at least one record of the occurrence of nodiiles of manganese- 
T^ J , ore in Indian waters. This is the one, noticed on 
Deep-sea nodules ' 
from the Maldive page 1116, in the channels between the composite 
atolls in the Maldive Islands. The depths given for 
soundings in such channels range from 100 to 769 fathoms, so that 
this occurrence can hardly be regarded as a deep-sea one. 
1 Probably to be explained as due to the supersaturation of the solution as regards 
the substances being deposited, here oxides of manganese and iron. 
