26 
POPULAR SCIENCE REVIEW. 
the notion that the agates of Mount Carmel were petrified 
melons, was seriously entertained by a writer of only a century 
and a half ago.* * * § Frequently the siliceous nodules are so 
abundant that the rock does not look unlike a conglomerate, 
and some mineralogists have even supposed that the agates are 
really pebbles and the enclosing rock nothing but the cementing 
material.f 
Although it is almost universally admitted that the vesicular 
cavities now occupied by the agates have originated in the 
manner described above, it is only fair to remark that a few 
Neptunists, unwilling to attribute an igneous origin to basalt 
and melaphyre, have sought to explain the formation of the 
cavities by assuming that they represent crystals in a porphy- 
ritic rock, which have been removed in solution, thus leaving 
angular hollows, the walls of which have since been rounded 
and otherwise modified by various solvents which have gained 
access to the cavities. J 
To explain the formation of an agate, with its concentric 
layers of chalcedony, jasper, quartz, and other siliceous minerals, 
is by no means so easy a matter as may at first sight appear. 
Not that there is any difficulty in getting the needful supply of 
silica to form the agate. It may be fairly assumed that some 
of the component minerals of the agate-bearing rocks will 
suffer decomposition by meteoric waters holding carbonic acid 
gas in solution, and that among the products of decomposition 
free silica will be found. It is notable that the more altered 
the rock, the finer the agates it contains ; thus suggesting some 
relation between the destruction of the rock and the construction 
of the agates. We have seen that labradorite is a constant con- 
stituent of melaphyre ; and this is, of all felspars, the most 
prone to alteration. Acted on by carbonated waters the silicate 
of calcium is decomposed, and a carbonate formed, whilst silica 
is set free. It should be remembered that the siliceous minerals 
in an agate are often accompanied by carbonate of calcium and 
by various zeolitic minerals. A large crystal of calcite may fre- 
quently be seen seated in the drusy interior of an agate-geode 
and it has even been suggested that the so-called “ fortification 
agate ” may owe its angularity of outline, as seen in section, to 
the deposition of silica, either upon or in the place of pre- 
existing crystals of calcite or of some zeolite.§ 
* Breve : “ Epistola de Melonibus petrefactis Montis Carmel.” 1722. 
t Volger : “ Studien zur Entwickelungsgeschichte der Mmeralien,”" 
p. 533. 
X See Bischof: “Lehrbuch d. Chem. u. Phys. Geol.” 1866, Bd. iii. 
p. 620. 
§ “ On Quartz, Chalcedony, Agate, Flint, Chert, Jasper, and other Forms 
