368 SW. Wallace—Geodes” of the Keokuk Formation. 
the soft deposits. They also often grew in fixed positions, as 
they are found crowded and fitting together in beds, wit 
angular forms. It is evident that only those which became 
covered by deposits would leave any remains in the strata. 
There is a great variety of forms and markings, but they are 
mainly of a few general and related types. The principal 
type is that of a massive peculiar cushion-like figure, with in- 
are similar, but the top is usually distinct from the lateral sides. 
They are frequently nearly round, sometimes higher than wide, 
but usually have three different diameters, the shortest of 
which is vertical, and so form a flattened oval. 
_ There are indications that the structure was fibrous; the 
fibers mainly running conformable to the surface and to the 
a large crinoid stem. It has split the column in five parts, bend- 
ing them apart to fit its form, into which they are imbedded up 
the sides. This is well preserved and curious. ; 
lany fine as well as large specimens. exist in collections 
here, and enough for many car-loads have been ship away. 
The largest, showing the outside markings finely, is owned by 
R. F. Bower. It is twenty-six inches across. A larger one, 
| generally arises 
mineral sponges, or from their having been silicified before 
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