18793 Monroe County Geology 



geode occupies the space left vacant by the dis- 

 solution of some organic object — a sponge perhaps, 

 or a shell, or the head of a crinoid. If a shell or 

 crinoid, the original shape is maintained; but most 

 geodes, probably having replaced sponges, are with- 

 out dehnite form. Broken open, they are found to 

 be hollow. Those that have a minute hole through 

 the rough crust are lined inside with chalcedony, 

 — that is, clouded quartz arranged in layers, ■ — a 

 peculiar structure caused by relatively rapid evapo- 

 ration. But when the crust is solid, the siliceous 

 liquid has evaporated very slowly, leaving the in- 

 side filled with more or less perfect crystals, usually 

 of white quartz, sometimes of amethyst — which 

 is violet quartz — sometimes mixed with crystals 

 of zinc blende, often of calcspar, and occasionally of 

 other minerals. Very rarely, a geode still retains 

 some of the siliceous water from the evaporation of 

 which it has been formed. 



The center of geode deposit is along the Mississippi 

 River about Keokuk, Iowa. At Niota, Illinois, 

 across from Keokuk, I once found specimens filled 

 with bitumen, but otherwise perfect. 



Monroe County is rich in fossils, also, and has 

 occasional caves worn by water in the limestone. 

 It possesses one special botanical charm. Arbutus rraUing 

 Hill, a barren, wooded slope covered in spring with ^"^"'"-^ 

 flowers of the fragrant Trailing Arbutus — Epigcsa 

 repens — and, as far as I know, the westernmost 

 point of its distribution. This was the choicest dis- 

 covery of our colleague, Herman B. Boisen, pro- 

 fessor of German, my own closest associate in the 

 old university faculty. Boisen was a warm-hearted, Bo 

 generous, enthusiastic Germanized Dane from Hol- 



C 199 H 



ioisen 



