71 
they were originally accumulated. I concluded that Fora- 
minifera had doubtless been present in them also, but that 
their calcareous shells had been dissolved out of them, and 
that this disappearance had been effected through the 
agency of water containing carbonic acid, at an early stage 
of the formation of these deposits. As is well known, this 
latter theory has been reproduced as a new one by some of 
the naturalists of the Challenger expedition, who have 
applied it to the explanation of phenomena of a substantially 
similar nature to those which I endeavoured to account for, 
in the same way, more than thirty years previously. 
I am indebted for the slab of limestone forming the sub- 
ject of this communication, to my friends the Messrs. Pat- 
tison, the marble merchants of Oxford-street, Manchester. 
This slab appears to illustrate in an exquisite manner both 
the theories to which I have just referred. It is a specimen 
of the Bolland limestone, which, when sawn through, was 
found to contain a large concamerated Nautiloid shell more 
than 12 inches in diameter, which appears to me to have 
been a true Nautilus, though the section has not passed 
exactly through its centre so as to reveal any portion of its 
siphuncle. In the various parts of this slab we find the 
calcareous material exhibiting different conditions. Through- 
out the greater part of its substance we have evidence that 
it has originated in an accumulation of minute calcareous 
organisms — especially Foraminifera* — but most of these are 
disintegrated and display vague outlines, a condition which 
I presume has resulted from the action of the carbonic acid. 
already alluded to. 
Scattered through the slab are numerous dark-coloured 
