F. M. Van Tuyl — Geodes of the Keokuk Beds. 37 



all geodes, and the same order of deposition may not obtain in 

 two adjacent specimens. 



For the purpose of illustrating the variations in the succes- 

 sion of the primary minerals in the geodes, the order of deposi- 

 tion in a number of typical specimens is given. The chalcedony 

 of the shell is listed first in each case. 



1. Chalcedony, quartz. 



2. Chalcedony, quartz, chalcedony. 



3. Chalcedony, quartz, chalcedony, quartz, ehalcedony, 



pyrite, calcite with included pyrite. 



4. Chalcedony, quartz, chalcedony, pyrite, calcite. 



5. Chalcedony, quartz, chalcedony, pj'rite, sphalerite. 



6. Chalcedony, quartz, dolomite, calcite. 



7. Chalcedony, quartz, magnetite, hematite 



8. Chalcedony, calcite, calcite. 



9. Chalcedony, calcite, millerite. 



10. Chalcedony, quartz, ankerite, calcite, aragonite. 



11. Chalcedony, quartz, calcite, bitumen. 



12. Chalcedony, calcite with included chalcopyrite. 



13. Chalcedony, quartz, pyrite, magnetite. 



14. Chalcedony, chalcedony, sphalerite. 



First, then, in the development of the siliceous geodes there 

 was formed a thin chalcedonic shell. Upon this is superim- 

 posed quartz, either in the crystalline or chalcedonic condition, 

 or calcite. It is a remarkable fact that when calcite and quartz 

 appear in the same specimen the calcite is normally subsequent 

 to the quartz, which rests directly on the siliceous shell. This 

 relationship, first pointed out by Professor Brush,* has been 

 found to hold in every instance by the writer, but A. H. 

 "Worthen claims to have found a single specimen at Keokuk 

 " in which large crystals of calcite are partly covered with 

 smaller crystals of quartz. "f 



The alternation of crystalline quartz and chalcedony in some 

 of the geodes is difficult to account for. If the layers were all 

 formed during one period of growth as seems probable, changes 

 in the condition and amount of silica supplied may have given 

 rise to the phenomenon. Changes in temperature or pressure 

 cannot be appealed to, because adjacent quartz geodes in the 

 strata frequently do not show the same alternations. 



The position of calcite in the geodes is subject to many varia- 

 tions. A.t times it succeeds the chalcedonic shell directly, but 

 more often it rests upon an inner lining of quartz or chalce- 

 dony. In some of the geodes, calcite of two generations ap- 

 pears. The earlier calcite is often discolored brownish, and is 

 frequently associated with or directly followed by sphalerite, 



* Geological Survey of Illinois, vol. i, p. 90, 1866. 

 f Idem, p. 90. 



