6 
Field Museum of Natural History 
be made up of successive layers deposited on the walls 
of the interiors of cavities, each layer toward the in- 
terior being younger than the one preceding. Some- 
times the process of deposition appears to have con- 
tinued until the cavity was entirely filled, but in other 
cases a vacancy still remains at the center. If the 
above is the method of formation, it is difficult to un- 
derstand why the deposition of the first layer, or, at 
least of the first two or three layers, would not close 
the cavity to succeeding deposits. 
Various attempts have been made to answer this 
question. Haidinger, a German geologist, writing 
about 1849, made the suggestion that the moisture 
ordinarily found in rocks, the so-called "mountain 
moisture" would "sweat" through into the cavities and 
that successive solutions of silica would thus enter 
through diffusion. This explanation seemed adequate 
to many investigators, but others have agreed with 
Noggerath, a contemporary of Haidinger, that it is 
doubtful if solutions would continually enter the cavity 
in this manner, especially as the outer layers of agate 
nodules are known by agate cutters to be particularly 
hard and impervious to liquids. As a better explana- 
tion Noeggerath called attention to an apparent canal 
or conduit which can be seen leading outwards from 
the interior of most agates and which he believed re- 
mained open during the formation of the agate for the 
admission of percolating waters. Such a channel is 
shown in Plate II. In some agates several such so- 
called entrance canals are to be seen, but in some, 
unfortunately for the theory, none can be found. 
Moreover, it is difficult to understand why such canals, 
if they ever existed, would remain open. The above 
theories, however, are the only ones that until recently 
have seemed at all worthy of credence as possible ex- 
planations of the manner of formation of agates and 
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