AGATES AND AGATE-WORKING. 
29 
that by the action of intermittent thermal springs the cavities 
in the amygdaloidal rock have been alternately filled and 
emptied. The solution carried upwards would certainly be 
more concentrated, and probably more highly coloured than 
the descending solution, from which much of the silica had of 
course been separated during its sojourn in the cavity ; hence 
the differences in the characters of the different layers. Not 
altogether satisfied with this theory, Herr Lange, of Idar, has 
suggested some modifications which he conceives will help to 
account for the regular deposition of thin layers lining the walls 
of the cavity.* He supposes that after gelatinous silica has 
been precipitated on the floor of a cavity, an accession of tem- 
perature causes the water to boil, and the pressure of the steam 
then forces the jelly in all directions against the walls of the 
enclosed space. If the tension of the steam become too great, 
it forces an exit by piercing the shell ; hence what we commonly 
call inlets of infiltration may after all be exactly the reverse — 
eruptive, instead of irruptive, canals ; channels of egress rather 
than of ingress. 
Both Reusch and Lange argue in favour of the deposition of 
silica from heated solutions. Great, without doubt, is the 
solvent action of water at a high temperature, and great the 
proportion of silica capable of being thus held in solution, as. 
testified by the vast mass of siliceous sinter deposited by the 
hot springs of Iceland, Colorado, and New Zealand. It has 
been well established too by the experiments of M. Daubree,f 
that certain silicates are readily attacked by water at high tem- 
perature and pressure, and suffer decomposition with separation 
of silica. But, on the other hand, many excellent chemical 
geologists are satisfied with the feebler power of cold water, and 
see a sufficient cause of agate-making in the slow but ceaseless 
action of meteoric waters draining through the rock, decom- 
posing the component silicates, and depositing free silica. Such 
action must necessarily be slow ; so slow, in fact, that, according 
to Bischofs estimate, the deposition of a layer one line in thick- 
ness requires twenty-one years. In order to form one pound of 
a methyst at least 10,000 lbs. of water must have~be^Hnfro- 
duced into the cavity and evaporated; an action which has 
been estimated to occupy the vast period of 1,296,000 years.J 
But this represents the formation of only a small stone, whilst 
in some parts of the world agates of gigantic size have been 
brought to light. Thus, an agate weighing a centner (110 lbs.) 
* “ Die Halbedelsteine aus der Familie der Quarze.” Von G. Lange, 
1868, p. 17. 
t (i Etudes sur le Metamorphisme,” 1860, p. 89. 
J u Lehrbuch d. Chem. Geol.” vol. iii. p. 636. 
