257 
of Edinburgh, Session 1876 - 77 . 
varies with the clay or ooze in which it was formed. Those from 
a fine clay usually adhere well together; those from a globige- 
rina ooze have an areolar appearance; those from a clay with 
many fine sandy particles usually fall to pieces. Mr Buchanan 
informs me that the purest portions of these nodules, that is those 
portions made up of closely packed concentric layers, contain from 
30 to 34 per cent, of the peroxide. 
Taking the nodule as a whole, it will of course contain very much 
less than this. The nucleus varies in each nodule, 'and that part of 
a nodule which is made up of concentric layers will vary with each 
locality, and with the depth from which it comes. We may expect 
therefore that analysis will show considerable variations in the 
amount of alumina, silica, and metals, lime, &c., in the nodules 
from different stations. At some places in the Pacific the nodules 
show periods of deposition very distinctly. We have first a very 
compact nodule which may have a shark’s tooth for a nucleus, and 
which appears to have been formed slowly. Then there would 
seem to have been a shower of ashes. After a time manganese 
was again deposited, inclosing in the nodule a layer of these ashes. 
The most frequent nucleus in the nodules is a piece of pumice or 
other volcanic fragment. 
In deep-sea clays, far from land, sharks’ teeth, ear-bones of whales, 
and fragments of other bones are very often the nucleus around 
which the manganese is deposited. In one instance a piece of 
siliceous sponge forms the nucleus. In a globigerina ooze a por- 
tion of the deposit has apparently formed the nucleus. In these 
we have perfect casts of the foraminifera, but all the carbonate of 
lime has been removed. The volcanic fragments which have 
formed the nuclei of nodules appear frequently to have undergone 
peculiar alterations. For instance, obsidian is usually surrounded 
by beautiful agate bands. 
When we found the bottom composed almost entirely of volcanic 
ashes, or so hard from other reasons that the sounding tube did not 
penetrate it, the manganese was deposited in layers over the 
bottom itself. Large pieces of this nature were taken several times. 
The escape of carbonic acid through the floor of the ocean near 
volcanic islands may in these regions greatly accelerate the pro- 
cesses which end in the deposition of the peroxide of manganese, 
