S. W. Wallace—“ Geodes” of the Keokuk Formation. 367 
best quarry rock of the Keokuk Formation. Stratification 
marks often show over and around the es similar to those 
around bodies in mud banks formed by currents. The shale 
seems to be but the remains of original deposits several times 
thicker, which have evidently béen dissolved out, leaving the 
insoluble portion to be compressed to the present shale. e 
limestone portions have not been compressed so much. 
The geodes themselves are merely crystalline shells formed 
from percolating water around the walls of vacant cavities. 
The outer shell is silica, generally chalcedony, with crystals of 
various minerals, principally silica and calcite, pointed inward in 
great variety and beauty. The external forms are sharply 
t. . 
__ The Indiana Geological Report, 1873, 278, gives the 
idea that they owe their origin to animal remains. This is 
evident by the peculiar family likeness through a great variety 
of sizes and forms; and by the lack of any other cause for 
them in such remarkable numbers, shapes and sizes. 
- An extended study of thousands of specimens and exposures, 
by the writer, confirms this by the recognition of peculiarities 
of growth and nature. It seems that the cavities were formed by 
the rotting out of sponges which had become covered by de- 
Sponges of trade, but without stems or apparent means of 
attachment. They may have of ‘ 
drifting along on the soft bottom, and often became covered in 
