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or less coloured strata, as shown by Fig. 3. It is only in 
some cases that this sort of structure is well seen by trans- 
mitted light ; but when a thin slice is examined by reflected 
light, it is seen much more frequently, owing to the circum- 
stance that many, in which it is really present, are occupied 
with colourless agate, and, under the former condition, do 
not, therefore, exhibit their component layers. In some few 
rare instances there is a vacant space left in the centre, 
owing to this last stage of agatization not having occurred ; 
and often the whole is filled with homogeneous agate. There 
are also cases where there is more or less of granular 
impurity in them, either disseminated or collected together. 
But not only are these reniform bodies converted into 
agate, but also very frequently are entirely filled with cal- 
careous spar. The difference between agate and calcareous 
spar is very readily discoverable by their action on polarized 
light. Calcareous spar, owing to its very intense double 
refraction, does not give rise to colours in the same manner 
as quartz or agate, in which the double-refracting power is 
so much less; but, unless the slice be extremely thin, 
except when we see portions of its system of rings, 
the only action which it has on polarized light, is to 
depolarise it in some positions as white light, and, by 
rotation, definite neutral axes are found. Thus, if the 
whole of the reniform body is filled with calcareous spar, 
having the crystalline particles all arranged in one direction, 
by rotating it round the beam of polarized light, with the 
analyser in such a position as entirely to suppress it, in some 
positions white light will be seen to pass through the whole, 
while in others the body will appear entirely dark. In 
other cases the bodies are filled with calcareous spar, which 
has been crystallized in portions that have their neutral axes 
inclined to one another at various angles, which gives rise 
to an appearance similar to that seen in Fig. 4 ; the dark or 
