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the interiors of the geodes must have proceeded in the belt of cementation. 
Most of these minerals are believed to have been deposited during the interval 
which just preceded the Pennsylvanian inundation but some are unquestionably 
much younger than this. 
The minerals now found lining the geodes were derived from the rocks im- 
mediately adjacent. Circulation could not have been great enough to introduce 
appreciable amounts of material from other formations, nor is it necessary to 
attribute an exotic origin to the minerals. Shale and impure limestone may 
contain any number of different mineral particles and the mineral species of 
the geodes merely represent the segregation of material through the medium 
of aqueous solutions from such disseminated particles. 
BIBLIOGRAPHY. 
1. Bassler, R. S. (1908) The Formation of Geodes with Remarks on the Silicifica- 
tion of Fossils, U. S. Nat. Mus. Proc., vol. 35, pp. 133-154. 
2. Benge, E. (1897) Geodes and Geodic Formation. The Mineral Collector, vol. 4, 
Nos. 7, 8, 9, 10. 
3. Dana, J. D. (1895) Manual of Geology, (fourth edition) pp. 97, 98. 
4. Shaler, N. S. (1899) Formation of Dikes and Veins. Geol. Soc; Am. Bull., vol. 
10, pp. 253-262. 
5. Wallace, S. J. (1878) Am. Jour. Sci. 3d series vol. 15, pp. 366-370. 
