198 
which, without further examination, would naturally be 
thought to be merely sand, and such it has hitherto been 
considered. When, however, examined with a microscope, 
it is at once seen to contain a very large quantity of reni- 
form bodies, which are evidently not sand, but some kind of 
minute organisms converted into agate. The mere occur- 
rence of minute agatized shells in this deposit would certainly 
not be a fact worthy of being laid before this Society, but 
since they exist in such vast numbers as to constitute a very 
considerable portion of a well-known rock, and, by the man- 
ner in which they occur, are presented to us in a form that 
is additionally interesting, perhaps a detailed account of them 
will not be thought a subject unworthy of attention. 
The chemical composition of a specimen containing no 
large shells was determined by dissolving in it hydrochloric 
acid ; and the fine muddy matter was separated from the 
coarser sandy portion by suspension in water and decanta- 
tion; from which I found that we had in 100 parts — 
Matter soluble in hydrochloric acid, being chiefly 
calcareous 60 
Coarse sandy matter 34 
Fine sandy matter 6 
100 
This coarse matter consisted, in nearly equal proportions, of 
the reniform agatized bodies and sand, whose particles varied 
in size from ii 5 th to ^ooth of an inch, the average being 
sJoth, of which the great bulk was composed. The fine 
matter contained some few grains of 7 ioth to iuooth of an 
inch, and all sizes down to T5o!o<5oth or less, but the bulk 
was made up of those of from xo.iooth to ^ smooth °f an 
inch. Hence it is clear that this deposit was not subjected 
to much washing at the time of its deposition, or else it 
would not have been composed of particles of such very 
varying magnitude. Although in some parts the coarse 
