199 
sand and reniforra bodies occur in such numbers as to 
touch one another, and although some portions of the grit 
retain their figure when the calcareous matter is dissolved 
away, yet in others it may be seen that a considerable part 
of it was deposited at the same time, and was not merely 
infiltrated among the coarser particles subsequently, although 
much of it has undoubtedly been introduced in this manner. 
If a thin slice of the stone be prepared, not much thicker 
than i^ooth of an inch, and examined with a microscope, 
the manner in which the grains of sand and the reniform 
bodies are imbedded in the impure calcareous deposit is 
well seen. The form of these bodies, however, is best 
made out by examining them as left after dissolving a 
portion of the stone in acid, when it will be seen that they 
vary somewhat in their shape. In many cases they are of 
a form that may most truly be termed reniform, — one side 
being perfectly rounded, and the other having an incurved 
depression of varying depth ; in some so slight that the body 
is almost globular, as shown by Fig. 1, Plate IV., which, as 
well as all the rest, except Fig. 8, is magnified about 200 times 
linear. When turned round, they are seen to be more or 
less depressed, and to have an oval form, both sides being 
symmetrical, as shown by Fig. 2. Their size varies from 
T }- 0 th to s&otii of an inch, the average being about sJotb. 
Their internal structure is best seen by mounting them in 
Canada balsam, which, having nearly the same refractive 
power as the agate, causes the light to pass through them 
as regularly as if we had a thin section. I have also care- 
fully investigated it by examining them in the latter con- 
dition, as occurring in a thin slice of the stone. By these 
means I find that they often have a structure similar to that 
frequently seen in large agates, the deposit having begun 
from the sides, and left a vacancy in the centre, which was 
afterwards filled up with less pure agate in alternating, more 
VOL. III. o 2 
