136 



PROCEEDINGS OF THE NATIONAL MUSEUM. 



VOL. XXXV. 



found lining the cavities. Dana, in his Manual of Geology, says these 

 geodes have been supposed to occupy the center of fossil sponges that 

 were at some time hollowed out by siliceous solutions and then lined 

 with crystals by deposition from the same or some other mineral solu- 

 tion. The writer has been unable to secure a full set of geodes from 

 the Keokuk bed to illustrate their method of formation, but the speci- 

 mens figured on the plates here presented are from shales and thin 

 limestones of a horizon in Kentucky not strictly equivalent but similar 

 in geological conditions. 



These geodiferous shales and thin limestones belong to the Knob- 

 stone division of the lower Carboniferous or Mississippian period. 

 Their strata cap the rounded hills of Indiana and Kentucky, encir- 

 cling that portion of these two States immediately underlaid by 

 Devonian and Silurian rocks. Southern Indiana, in the vicinity of 

 New Albany, and the " Knob " region of Kentucky, extending south 



Sketch showing Occurrence of Geodes in Knobstone Shales. 



and then southeast of Louisville, give many exposures of the Knob- 

 stone group. Button Mold Knob, several miles south of Louisville 

 and just opposite St. Jacobs Park of that city, is a celebrated locality 

 for Knobstone fossils, and here the writer made most of his collections 

 and observations. The geodes described in this paper from Button 

 Mold Knob occurred in the lowest division of the Knobstone, a series 

 of blue shale termed the New Providence shale. These strata are so 

 compact that water can not readily pass through them. A few thin, 

 ferruginous limestone layers are interbedded with the shale, but they 

 are so infrequent as to have little effect upon the porosity of the 

 strata. The rainfall upon these shales is, therefore, carried away 

 almost entirely by surface drainage, and in the case of level areas 

 underlaid by the shale, a damp, unproductive clay soil is the result 

 of this imperviousness to water. Still, these shales are often crowded 

 with geodes, objects which require the presence of water for their 

 formation. This apparent association of many geodes and no water 



