54 
Proceedings of the Boyal Society 
structural features which the individual objects possess, their mode 
of succession in linear directions perpendicular to the planes of the 
crystal displays a conformity to law, which could not have resulted 
from any chance deposition of coloured particles, whether solid or 
liquid, on the surface of the crystal during the process of forma- 
tion. They must therefore belong to some peculiar crystalline 
form, or to some order in the organic world. 
The mode of aggregation just described has obviously a close 
resemblance to some of those animal structures produced by con- 
tinuous gemmation, as for instance some of the compound Fora- 
minifera. 
But the agreement between the objects and organic bodies is 
not confined to form and other structural resemblances — it extends 
to the changes through which they must have passed before they 
were enclosed in the substance of the crystal. Whatever their ori- 
ginal nature, so completely have they become impregnated with the 
inorganic elements of the crystal, that the more opaque layers are 
often viewed through the silicified casts of their successors. Before, 
however, their condition became thus permanently fixed, there is 
evidence of continued and varied change. Specimens of amethyst 
contain whole-layers from which the upper or globular end of each 
object has entirely disappeared, and their interior become occupied 
with silica so transparent that the delicate structural features of 
the conical extremity can be equally well seen when viewed on 
either side by transmitted light. In other instances nothing 
remains except portions of concentric annuli. 
The objects that retain their structural features most entire are 
opaque, and as seen in a small section of dichroite appear of a 
brilliant white on the lower side, and of a somewhat silvery lustre 
on the upper. The same section also exhibits the changes on the 
external envelope, including its partial and complete removal. In 
one group it has disappeared from the one side, while it remains 
quite entire on the other, producing a well-marked boundary line, 
which passes over many of the individual objects, displaying in 
striking contrast the difference between the outside shell and the 
matter of the interior. Whenever divested of this covering, as is 
generally the case, the bodies are seen to be in groups of various 
colours, namely, red, orange, yellow, browns of various tints, and 
dark blue. 
