208 W. A. Tarr — Cone-in-Cone, 



was due to the growth of the concretion. If the concre- 

 tion actually developed in place a certain amount of pres- 

 sure would have developed. However, the writer 1 has 

 proved that certain concretions occurring with cone-in- 

 cone over them were syngenetic in origin, hence no pres- 

 sure would have developed in such cases. If it can be 

 assumed that the growing strength of a calcareous con- 

 cretion is equal to the crushing strength of similar mate- 

 rial, the pressure so produced would be considerable, but 

 would be dependent upon the compressibility of the 

 enclosing materials. That some pressure would be pro- 

 duced in the case of ' epigenetic concretions cannot be 

 denied, but some other source of pressure must be sought 

 when the cone-in-cone is not associated with concretions, 

 or when the concretions are syngenetic. The cone-in- 

 cone unassociated with concretions is as perfectly devel- 

 oped as when associated with them. This is especially 

 true of cone-in-cone in coal. 



Another possible source of pressure exists, if the 

 original form of the calcium carbonate was aragonite and 

 had been changed to calcite. This change involves a 

 volume increase of 8.35 per cent, sufficient to produce 

 considerable pressure. But is there any reason for 

 assuming that the original form was aragonite! No 

 cone-in-cone the writer has examined is composed of 

 aragonite now. This very fact, however, may mean that 

 cone-in-cone develops only where aragonite has altered 

 to calcite as will be discussed below. All cone-in-cone 

 structures show fibrous calcite, but that this was origin- 

 ally aragonite cannot be proved. Fibrous calcite from 

 Boone Co., Missouri has been sold to mineral dealers as 

 aragonite, showing that this structure was regarded as 

 evidence of the material being aragonite. The develop- 

 ment of cone-in-cone only in a crystalline, fibrous calcite 

 is significant, however, and certainly suggests that arago- 

 nite . was the original . form of the calcium carbonate. 

 According to Clarke 2 " aragonite alters easily to para- 

 morphs of calcite, but the reverse change rarely, if ever, 

 occurs/' Calcite is the more stable form. 



It does not seem likely that either the growth of concre- 

 tions or the weight of overlying sediments would be an 



1 Tarr, W. A. : Syngenetic Origin of Concretions in Shales, Bull. Geol. 

 Soc. Am., vol. 32, pp. 373-384, 1921. 



2 Clarke, F. W., Data of Geochemistry, U. S. G. S., Bull. 695, p. 549. 



