201 
lighter divisions, of course, changing in intensity by rotation, 
but retaining their forms, which are angular, and such as 
would be produced by the infiltration of calcareous spar into 
a shell. When, however, they are filled with agate, no such 
effect is produced, but in every position we have the appear- 
ance of a white disk, with black bands radiating in an 
irregular manner from the centre, (as shown by Fig, 5,) 
and changing their forms and places by rotation, whilst the 
intermediate parts have patches of orange or blue, which 
properties agree with what would result from such an 
irregular structure related to the centre as the infiltration 
of agate would most probably produce. In some cases, 
one portion is converted into agate, whilst another is cal- 
careous, sometimes in the fprm of crystalline spar, and at 
others as a concretion, similar to those sometimes to be 
seen in the substance of the shells from the grit, which 
are partially agatized. In these, some of the original cal- 
careous matter still remains in the form of small concretions, 
sometimes disseminated in the agate, but generally attached 
to the sides of the shell. In the same slice, and in the same 
field of view of the microscope, we find some of the reniform 
bodies filled with agate, and others with calcareous spar ; but 
their relative numbers vary in different parts, in some parts 
nearly all being agate, and in others calcareous. 
I counted in the space of ^l^th of a square inch, in an 
average portion of a thin slice, no less than forty of these 
bodies, and, therefore, on a square inch there would be 
11,200; and since they are, on an average, ^ioth of an inch 
in diameter, there would be at this rate about two and a 
quarter millions in a cubic inch. Moreover, taking into 
consideration those filled with calcareous spar as well as 
those that are agatized, I shall not be overrating them if I 
suppose them to constitute 20 per cent, of the whole rock. 
Calculating on this supposition, and on their being ^Joth 
