4 SMITHSONIAN INSTITUTION 



As illustrated in Figure 4, nos. 1-5, the crinoid buttons or indi- 

 vidual segments composing the column (no. 1) may have the central 

 canal filled with mineral matter deposited by circulating waters. Then 

 a number of segments still in position but fractured (no. 2) may have 

 the cracks enlarged, by such mineral matter (no. 3) and still more so 

 (no. 4) until there results a round, hollow ball which, when broken 

 (no. 5), shows the inwardly pointing crystals of a typical geode. 

 Silica is the principal mineral concerned in geode formation of many 

 areas, but the problem of replacement by silica or silicification is of 

 especial interest to the paleontologist because many beautiful calcare- 

 ous fossils buried deep in solid limestone and impossible of extraction 

 without marring their structure may be freed in perfect condition 

 through this process. In this case the organic calcite composing the 

 fossil is dissolved and replaced by silica, while the surrounding lime- 

 stone is taken into solution and removed by mineral-bearing waters 

 circulating through the rocks. It has been believed that such changes 

 by silicification required thousands of years, so that localities which 

 had been exhausted by present-day collecting could not be expected to 

 yield additional specimens for centuries to come. However, it was 

 shown on the present field trip that less than 25 years are required 

 to cause this change from the calcareous to siliceous condition in fos- 

 sils, when conditions are right. Fossils observed that long ago partly 

 buried in the solid rock and covered then with a thin layer of mud to 

 supply silica for the surface waters, were noted on the present visit 

 to have weathered out into free, completely silicified specimens. 



Leaving the Kentucky Knobs, strata of the same age in northern 

 Tennessee were next investigated. Here the outcrops are in glades, or 

 bare places along the hillsides, in which every layer could be traced 

 often for long distances (fig. 3). Certain of these layers rich in 

 crinoid fragments and which had been prepared for silicification by 

 the writer some years ago now yielded abundant results. Proceeding 

 then to western Tennessee, the writer investigated some of the dump 

 heaps of Doctor Springer's quarries in the Silurian formations of 

 Decatur and adjoining counties. Here again silicification and the usual 

 forms of weathering during the past 20 years had exposed some addi- 

 tional specimens of rare species. Among these is the crinoid Allocrinus, 

 with its heavy annplates and delicate column (fig. 4, no. 6) ; Euca- 

 lyptocrinus, with its tightly compressed arms (no. 7) ; Gnorimocrinus, 

 known hitherto only in Europe (no. 8) ; Gissocrinus, also a European 

 genus with peculiar broad plates (no. 9) ; Calceocrinus, with the crown 

 recumbent upon the column through a hinged articulation (no. 10) 

 and finally Myclodactylus, once mistaken for a coiled shell but really 

 a dainty crinoid in which the column encircles and protects the deli- 

 cate crown (nos. 11, 12). 



