4 
Field Museum of Natural History 
formed. From very minute, they run up to weights as 
high as 4,000 pounds. 
Although most agates are formed in the cavities 
of volcanic rocks, some are formed in other cavities, 
such, for instance, as those left by the decay of wood. 
The siliceous linings of rock fissures sometimes also 
have the structure of agate. The so-called agate nod- 
ules are not always composed entirely of agate. Fre- 
quently, quartz crystals or other forms of quartz or 
even other minerals line the interior of the nodules, 
or are interpolated with the layers. 
Occurrence of Agates 
Agates are found in many parts of the world, be- 
ing likely to be formed wherever there are trap 1 rocks. 
As already noted, decay of these rocks leaves the more 
resistant agate nodules in the soil, from which they 
are often distributed by streams. 
Agates resulting from the decomposition of the 
trap of the Deccan plateau in India have been gathered 
for thousands of years. After more or less fashioning 
by native lapidaries they have found their way to Eu- 
ropean and Oriental markets. A further description of 
this industry will be found in subsequent pages. The 
principal European locality for agates is in south- 
western Prussia in the region of the Nahe River, a 
tributary of the Rhine. Not only has the agate found 
in this region been worked for centuries, but the vari- 
ous processes of shaping and coloring it, have been so 
fully developed there that it is now the world center 
of the agate business. Among other European locali- 
ties, the so-called Scotch pebbles from Forfarshire 
and Perthshire in Scotland furnish attractive agates. 
The most extensive ocurrence of agate known at 
'Trap. A common name for any dark, finely crystalline, ig- 
neous rock. 
[108] 
