700 NOTES ON THE GEODES OF ILLINOIS. 
and other not yet mentioned ‘‘layers” of the crust are so called 
more for convenience, than because they exist sharply defined in 
every specimen, for such is not the case; in most they shade into 
each other by almost imperceptible degrees. The outer surface 
is generally light colored— either yellow, or drab, or yellowish 
brown; but when there is much oxide of iron present the color 
deepens to chestnut brown. The general form of the geodes is 
more or less spherical and in a majority of the specimens it is 
quite regularly so, but the character of the interior seems to have 
some influence in shaping the whole mass. In my own collecting, 
at least, it has uniformly been true that those geodes that con- 
tained only quartz were most regular in form and those lined 
with crystallized quartz are rather more regular than those lined 
with chaleedony. Those specimens that contain oxide of iron 
are often quite flat, as are those with calcite, though these latter 
are usually flattened more on one side than on the other and in 
various ways made irregular. 
Although sometimes packed almost as thickly as possible in the 
shale, the geodes are for the most part entirely distinct from each 
other; but sometimes two or more are found adhering to each 
other, either two of nearly equal size forming a dumb-bell shaped 
mass, or, more commonly, a large one is surrounded by several 
much smaller. These are not often so firmly attached that a 
sharp blow will not separate them entire. When broken these 
smaller geodes are usually solid. Sometimes small, pocket-like 
geodes are found in the crust of large and heavy ones, and some- 
times these extend so far over the surface that the geode becomes 
like a ball coated outside and in with crystals, with occasionally 
a thin clayey crust over a part of the outside. 
The geodes are not exactly alike in different portions of the 
region. In some places nearly all are small, while large ones are 
not infrequent in others, in one place most are regular spheres, in 
another most are quite irregular, in some parts of the bed they 
lie so thickly as to crowd each other, in other parts they are far 
apart. Not only do these and similar differences occur in places 
at some distance from each other, but in the same place upper, 
lower or middle portions of the bed may differ widely in the 
number, form, size and contents of the geodes. Everywhere 
many of the geodes are solid, and the first thing for a collector to 
learn is to judge by the weight of any given specimen whether it 
