202 
of an inch in diameter, and, therefore about u.oso.oocth of 
a cubic inch in content, there would be about three millions 
in a cubic inch. Hence I think it may be safely considered 
that there are two or three millions of them in a cubic inch 
of the stone, when it does not contain many larger shells. 
Having thus described these reniform bodies, and shown 
in what vast numbers they are found. I will consider what 
they are. That they cannot be grains of sand is quite cer- 
tain, when we compare their forms and optical properties. 
Grains of sand are usually considerably angular, and, when 
more rounded than is common in the rock under considera- 
tion, have a form similar to that shown by Fig. 7. When 
seen by reflected light, if mounted in Canada balsam, they 
are clear and transparent, and not milk white, like the 
reniform bodies, and never show anything like the agate 
structure seen in Fig. 3. Using transmitted polarized light, 
they show a definite single system of neutral axes, giving in 
some positions the even dark appearance shown by Fig. 7, 
and in others various simple colours of a low order, and 
never any radiating bands at all like those presented by the 
reniform bodies, (as shown by Fig, 5.) 
Those which are calcareous do not appear to be rounded 
grains of shell or coral, for they are, similarly, much too 
regular in their form. I have had it suggested to me that 
they might be calcareous concretions similar to the ovum-like 
ones of oolite rocks, those which are now agate having 
subsequently been converted into it. I, however, think that 
when the characters of oolitic grains are compared with them, 
there is very good evidence that they are not. In all the 
oolitic rocks which I have examined, the concretions are 
composed of alternating concentric layers of more or less 
impure calcareous matter, formed round grains of sand, 
fragments of shells or coral, or entire small shells, (as shown 
by Fig. 8.) On the contrary, that with which some of the 
