no. 1637. ON THE FORMATION OF GEODES— BASSLER, J4J 



therefore, only when fragments of the original shell are retained, as 

 in figs. 9 and 10 of Plate XXII, that no doubt as to the origin can 

 exist. In these brachiopods it will be noted that the main fracture line 

 is at right angles to and passes through the beak. From this line 

 other fractures often radiate, so that in case of removal of the inter- 

 vening calcareous shell matter a septarian like effect is produced 

 (Plate XXIII, fig. 11). 



In all of the specimens just described the deposition of silica in the 

 fracture has been preceded by a complete silicification of the fossil 

 itself, in the manner described on page 135. Sometimes this com- 

 plete replacement of calcite by silica leaves a perfect representation 

 of the fossil, but again silicification may destroy all the details of 

 structure (Plate XXIV, fig. 5). Should the fossil when silicified pre- 

 sent no fracture, no deposition of crystals preliminary to the forma- 

 tion of a geode occurs, but fractured silicified specimens are subject to 

 geodization. The column figured on Plate XXIV (fig. 6) is com- 

 pletely silicified and small stringers of quartz occupy the fracture. 

 Less frequently these two processes of silicification seem to occur 

 simultaneously. 



Returning to the suggestion in Dana's Manual of Geology that the 

 Keokuk geodes are hollowed out sponges lined with crystals, it seems 

 more reasonable, in view of the absence of such sponges in that forma- 

 tion and the presence of numerous specimens indicating the origin 

 described above, that the latter is nearer the truth. Professor Shaler\s 

 idea that this class of geodes is formed when deeply buried is not in 

 accord with the facts, at least as observed by the writer, nor does 

 there appear to be any necessity for the water of formation to be 

 under a considerable though variable pressure. Ordinary surface 

 waters charged with silica seem to be sufficient. The other objections 

 to Professor Shaler's views regarding the method employed by the 

 silex-bearing waters to reach the interior of the geode are explained 

 in tracing their origin to fractured fossils. The deposition of crystals 

 on opposite sides of a fracture will always leave a space free to the 

 passage of water, so that no necessity exists for this water of forma- 

 tion to penetrate the solid mass. 



In conclusion, the best known geodes are free, rounded, siliceous, 

 hollow masses lined internally with crystals, and occurring usually 

 in shaly, fossiliferous strata. Fossils lying in the path of surface 

 waters are subject to a complete replacement of their substance by 

 silica ; when crushed or fractured, fossils may have the breaks lined 

 with a siliceous deposit. In the latter case, continued deposition and 

 the expansive force of the crystals will result in a hollow, rounded 

 mass, or geode, lined with inwardly pointing crystals and on the out- 

 side covered with remnants of the fossil. The chemical side of this 

 subject is a problem which is not in the province of the present paper. 



