AGATES, CARNELIANS, AND JASPERS. 165 
made up of so many different layers, one outside the other, 
and were to cut it across, the whole would present the ap- 
pearance of a set of concentric rings. This is the mode of 
origin of the remarkable structures known as “ Eyes,” which 
give the distinguishing name to Hyed Agates, 
With a further growth of chalcedonic material around the 
eyes, the interspaces between two adjoining eyes gradually 
becomes filled up as in the case of the stalactites already 
described. The layers begin by conforming more or less to 
the shape of the surface upon which they are deposited ; but 
owing to the fact that more silica is deposited at those points 
where two adjoining surfaces come nearest into contact, the 
interspaces are gradually filled up. Now, if we examine a 
cross section of an agate which has grown up in this way, 
the variously-coloured layers show interesting patterns, 
which follow all the ins and outs of the surface, and present 
a series of sharp retiring angles on the side next the wall of 
the agate, and show advancing curves on the sides facing 
the centre. It will be noticed that after passing a certain 
point in tracing these bands inward towards the centre of 
the agate, the layers tend more and more to form regularly 
concave surfaces towards the centre. The influence of the 
outer irregularities may, however, be traceable almost up to 
the centre of the agate. It is by this process that Fortifica- 
tion Agates have been formed. 
It was mentioned above that the molecular constitution 
of the silica-jelly carried into the agate cavities in solution 
is by no means always alike. Sometimes it contains a few 
other substances in solution, and these may affect the colour 
of the final product, and may also give rise to some modi- 
fications in its comportment within the cavity in conformity 
with the well-known behaviour of solutions of different com- 
position in relation to Surface Energy. It is customary to 
speak of the chalky forms of Chalcedony as Cacholong, though 
the term is properly restricted to one of the forms of Opal. 
The other principal form which may be assumed by the 
silica jelly when it finally hardens is that known as Opal, 
which differs from Chalcedony in somewhat the same man- 
ner as Barley Sugar differs from Sugar Candy. The former 
