AGATES, CARNELIANS, AND JASPERS. 171 
not yet seen any case in which the Tube of Escape affects 
either the Clear Chalcedony Layer or any Opal bands. From 
this we are probably justified in concluding that both of 
these had consolidated before the expulsion of the silica 
jelly destined to form the chalcedonic part of the agate. 
In a few cases the cavity left by the expulsion of the 
silica remains empty, or becomes a druse, lined with either 
Quartz or Amethyst. But in most cases a later filling ensues, 
and gives rise to a second agate within the first. 
The factors then, which determine the arrangement of 
the layers within an agate may be grouped in two primary 
categories :—(1) Those which are contemporaneous with the 
srowth of the agate; (2) those which are of subsequent date. 
To the former belong Moss Agates, Eyed Agates, Stalactitic 
Agates, and Fortification Agates ; to the latter must be re- 
ferred all those disturbances which are caused in connection 
with the formation of the Tube of Escape. 
Amongst the later changes which affect agates a foremost 
place must be given to the effects of molecular rearrange- 
ment. Opal, and colloid substances in general, tend to pass 
into the crystalline state, just as glass with age becomes 
devitrified, or as the colloid Barley Sugar passes into the 
erystalline Sugar Candy. As already mentioned, part of 
the material of an agate which now behaves as Chalcedony 
has very probably at one time been Opal. In the passage 
from the one state to the other a certain amount of heat is 
liberated, equivalent to the difference between the specific 
heat in the colloid state as compared with that of the crystal- 
line. This may have had some effect upon a full agate in 
giving rise to the internal pressure above noted. But in 
another way it affects the agate because during the molecular 
rearrangement some of the impurities which may be present 
are enabled to separate out in various forms—commonly in the 
form of small ellipsoidal solids. Another result, arising at a 
later period from the same cause, is a small diminution in 
volume, consequent upon the loss of some of the water previously 
existing in chemical combination in the Opal. This often 
gives rise to short cracks transverse in direction to that of the 
layers. The same cause often favours a more advanced change 
