298 C. II Gordon— Keokuk Beds at Keokuk 



attributed them to the intrusion of water charged with sili- 

 ceous material from below. The accession of siliceous material 

 to the waters of that epoch, however caused, resulted in the 

 more or less complete extermination of the life characterizing 

 the preceding epoch. Upon the restoration of normal condi- 

 tions, however, a profusion of forms again appears though 

 marked by variations in type and form. This tendenc} 7 toward 

 extinction may be observed in the smaller as well as the more 

 important chert beds. The crinoidal fades of the first crinoid 

 bed is essentially different from that of the second, while that 

 of the third is unlike either. The destructive effect of excess 

 of siliceous material represented in the Geode bed is especially 

 noticeable in the paucity of species that were able to survive 

 it. The almost complete extinction of the Actinocrinidse is 

 especially remarkable.* Not only is this true of crinoidal life 

 but it has been remarked that a profusion of fish remains is 

 nearly always accompanied by greater or less quantities of 

 chert. The obvious connection existing between the presence 

 of the chert and the extinction of previously existing species 

 is certainly of great interest both from dynamical and evolu- 

 tionary standpoints, though at present little understood. 



At some of the partings in this division the upper surface 

 of the lower stratum shows decided evidences of erosion. The 

 limestone matrix has in some cases been worn away, leaving 

 the fossils projecting one-half to one inch above the general 

 surface. The rock-surface is usually, in these cases, much 

 discolored and altogether its appearance seems to indicate 

 changing submarine currents. 



The upper division or Geode bed consists principally of 

 argillaceous shale varying to an impure shaly magnesian lime- 

 stone. The lower portion is more calcareous, sometimes 

 including thin layers of tolerably pure limestone. An abun- 

 dance of siliceous matters occurs here in the form of goedes 

 which, with the exception of a few feet in the lower part, are 

 disseminated throughout the formation. They are generally 

 smaller and thicker in the middle and upper portions of the 

 bed, while in the more calcareous layers below the geodes 

 are large and contain more perfectly formed crystals. 



The formation contains three crinoid beds quite clearly' 

 distinguished from each other in the character of their 

 forms. Outside of these horizons crinoids sometimes occur, 

 but sparingly. 



The third bed, No. 13, of the section has not been found out- 

 side of a single locality in the lower part of the city discov- 

 ered by Mr. L. A. Cox. Most of the species obtained from 

 this bed were new* and belong to the PoteriocrinidEe, evi- 



* Keyes, American Naturalist, March, 1890, p. 243. 



