Conglomerates, and Marls of Devonshire 
27 
" The fii'st consists in the conversion of oxide of iron" (in the 
sediment) into sulphuret of the metal. My attention was first 
directed to this process by observing it actually in progress in the 
harbour of Pictou. This harbour receives the water of three rivers 
and several smaller streams, which in times of flood carry into it 
large quantities of reddish mud, which sometimes discolours the 
whole surface. This mud, with similar sediment from the shores of 
the harbour, is deposited on the bottom, and there undergoes a re- 
markable change of colour. A portion of old mud recently taken 
from the bottom is of a dark grey colour, and emits a strong smell 
of sulphuretted hydrogen. When dried it loses this odour, and its 
colour is a pui-e grey without any trace of red. If a piece of the 
dried mud be heated to incipient redness, it emits a sulphurous odour, 
and at once assumes the red colour which belonged to the sediment 
before it was deposited. It thus appears that the ii-on of the red clay 
has entered into combination with sulphur, and this is probably 
obtained from the sulphate contained in the sea water, by the de- 
oxidizing influence of decaying vegetable matter, the greater part of 
which seems to be furnished by the eel grass ( Zostera marina ) , which 
grows abundantly on the mud flats. It is evident that this modern 
deposit is quite analogous to many grey beds of antiquity, in which 
sulphuret of iron is mixed with organic matter, and there can be little 
danger in inferring that the causes in both cases are the same. I may 
mention, that in some parts of the deposit forming in Pictou harbour, 
the vegetable matter which has caused the change of colour is so 
completely decomposed that no visible fragment of it remains. 
* ' Another modem cause of the change of colour in red sands and 
clays is the action of acids, produced in the putrefaction and decay 
of moist vegetable matter. This is the usual cause of the whiteness 
of the subsoils of peat bogs and swamps, and in such places the 
oxide of iron is often re-deposited at the outlet where the surplus 
water escapes from the bog. This process also probably prevailed 
extensively in the freshwater deposits of former periods, and may 
have changed the colours of clay and sands, and have collected their 
colouring matter in bands and nodules of carbonate and hydrous 
peroxide of iron. Beds bleached in this way of course do not resume 
their colour when heated." 
In the conclusion of the paper the author draws some interesting 
inferences of a general nature, and states his belief that " the oc- 
currence of grey beds and patches in red formations may often 
indicate the former existence of fossils whose forms have perished." 
It can scarcely be doubted, that the origin of the yellowish spheres 
of the Conglomerates is in some way connected with the stony nuclei 
which they invariably contain ; it is difiicult, however, to understand 
* " Quart. Journ. Geol. Soc," vol. v, page 25, <S c. 
