212 GENEKAL GEOLOGY. 



specimens in miner alogical cabinets as to merit a brief notice. 

 They are more or less spherical masses of silex, usually 

 hollow and lined with crystals of quartz. The onter crnst is 

 rough and unsightly, but the crystals which stud the interior 

 cavity are often very beautiful. The prevailing kinds of 

 crystals are of quartz, but those of calcite are quite common 

 in the same geode. Sometimes they also contain crystals of 

 the sulphurets of iron and zinc. They vary in size from that 

 of a walnut to a foot in diameter. The crust of the geodes in 

 this formation is invariably silicious, but geodes are found in 

 the soft magnesian limestone of Devonian age in Bremer 

 county, the crust and lining crystals of which are carbonate of 

 lime, some of which are quite free from silex. 



The Geode bed of the Keokuk limestone seems to be some- 

 what local in its development as such, because it is not 

 recognizable in the northern portion of the formation in 

 Iowa, nor in connection with it when it rises again, seventy- 

 five or eighty miles below Keokuk. 



Economic Value. The economic value of this formation 

 consists entirely in its stone, but this is very great. Some of 

 its layers furnish the finest quality of building material yet 

 found in the State; large quantities of which have been 

 carried to distant points for the erection of costly structures, 

 among which is the Custom House and Post Office building at 

 Dubuque, and a part of that also at Des Moines. The 

 principal quarries of it are upon both banks of the Missis- 

 sippi river from Keokuk up to Nauvoo, a distance of 

 about fifteen miles. Many quarries are also worked within 

 the city of Keokuk. 



The accompanying sketch of one of them in the southern 

 part of town will give an idea of the stratification and general 

 aspect of such exposures. 



Notwithstanding the great value of much of the stone of 

 this formation, care is requisite in selecting building rock 

 from it, because some of the layers, although they look well 

 when first quarried, will, nevertheless, split and crumble to 

 pieces upon exposure to the atmosphere and frost much in 



