Agate —Physical Properties and Origin 
9 
the beginning of the formation of a zone of precipi- 
tated silver chromate. To the neighborhood of this 
zone as shown in c, all the super-saturated silver 
chromate wanders, leaving a colorless area. But be- 
low the chromate-free area a new zone of dissolved 
silver chromate has formed as shown in d. Ere this 
has had time to pass to the zone of deposit, its con- 
centration has become so high that within its own area 
deposition takes place as shown in e. By the gather- 
ing of the super-saturated silver chromate at this edge, 
a second, colorless zone is formed as shown in /. Sim- 
ilarly, a third zone of deposit is formed as shown in g 
and so the process goes on until a series of bands is 
produced. These bands are parallel to the original 
outline of the colloid and are of wonderful regularity. 
Their appearance is marvellously like that of the bands 
of a fortification agate. An example of banding pro- 
duced in this way is shown in Plate III. 
Besides explaining the banding, this view of the 
method of formation of agates also indicates why agate 
nodules are often hollow in the interior. Drying of 
the colloidal silica causes a shrinking in bulk which 
would often leave such a hollow. The theory also 
explains the frequent occurrence of quartz crystals at 
the interior of agates. Crystals cannot form in col- 
loids on account of surface tension, but when the ten- 
sion is relieved at the hollow interior of the nodule, 
complete crystallization can take place there. 
Anyone wishing to illustrate for himself the for- 
mation of the bands referred to can readily do so by 
making the following experiment: Dissolve 3 grams 
of gelatine in 60 c. c. of water, add 3 drops of a 5% 
citric acid solution and 4 drops of a 10% ammonium 
bi-chromate solution. Stir the liquid and pour some 
of it on a clean glass plate. Allow it to harden for 
one to two hours. Then with a glass rod that has been 
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