514 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



only a thin layer enclosing a nodule of amethyst, as already described under amethyst. One 

 accordingly distinguishes agate-anngdales and amethyst-amygdales, though the difference is 

 only one of proportion. In many cases the innermost layer of an amygdale is constituted 

 by crystals of calcite or other minerals, especially the hydrated silicates belonging to the 

 zeolite group, which rest upon the crystals of amethyst. 



Sections of agate-amygdales often disclose other peculiarities which are important, 

 inasmuch as they throw light upon the mode of formation of the amygdale. Thus it has 

 been observed that the central cavity of the amygdale is placed in communication with the 

 exterior by a narrow canal, formed by the layers of agate bending sharply outwards at this 

 particular point. At least one such canal, and very frequently several, exist in every 

 amygdale. 



The external opening of the channel is frequently to be seen on the surface of the 

 amygdale as a funnel-shaped depression, but is sometimes obscured and only to be made out 

 in sections. These canals may be empty or, on the other hand, completely filled up with 

 agate of the same description as that of the innermost layer of the amygdale, being indeed 

 continuous with this. 



An explanation of the origin of agate-amygdales, sufficient to account for all their 

 details of structure, can be deduced without much difficulty from the general character and 

 the mode of occurrence of these structures. First, however, must be noted that the 

 amygdaloidal cavities, in which the agate occurs, are found in the rocks already mentioned 

 only when these have been much weathered and their constituents partly removed. The 

 more advanced the decomposition of the rock — which may indeed proceed as far as almost 

 complete disintegration — the more likely is it that the amygdaloidal cavities will be filled 

 with mineral. On the other hand, the less decomposed is the rock the less likely is 

 agate to be found in its cavities, these cavities in fresh unaltered rock being always 

 quite empty. 



It follows from this, therefore, that the material which fills the cavities, namely, the 

 silica, has been derived from the rock in which they are enclosed. During the ])rocess of 

 weathering, the silica and other constituents of the rock have been dissolved out by water 

 and subsequently redeposited in the cavities. The banded structure of the amygdale, 

 however, indicates that the deposition did not proceed continuously and uninterruptedly, 

 but that between the formation of two successive layers there was an interval of greater or 

 less duration. 



To explain this phenomenon the existence is assumed of hot intermittent springs, such 

 as are now seen to perfection in the geysers of Iceland and in the United States National 

 Park on the Yellowstone river. The essential condition for the formation of amygdales is 

 that the hot or warm water rising up from the depths shall saturate the rocks, and that it 

 shall sink again, leaving the rocks dry for a period. The hot water dissolves out the silica 

 and other constituents of the rock, and the solution fills up the amygdaloidal cavities. 

 When the waters sink these cavities are emptied, only a film of water covering their walls 

 being left behind. On the evaporation of this film, which readily takes place at such a high 

 temperature, a thin layer of silica is deposited on the walls of the cavity. When the hot 

 spring again rises the same thing takes place and a second layer of silica is deposited, and 

 so on, until the cavity is more or less completely filled up. Each time the fluid passes 

 into and out of the cavity by the canals mentioned above, which are for this reason often 

 referred to as tubes of entry or escape, the fluid also perhaps passes to a certain extent 

 through the porous agate itself. Crevices, fissures, and other cavities in the rock are, of 

 course, filled with agate in the same way. 



