no. 1637. ON THE FORMATION OF GE0DES—BAS8LER. 137 



led to a study of the local conditions, with the result that the geodes 

 were found to occur only in zones or areas reached by surface waters. 

 Usually the geodes were lying on the surface itself, free or partially 

 covered by the soil, and digging in the compact shale immediately 

 beneath them would reveal no trace of other specimens. Tn other 

 cases they were apparently buried in the shale, but, in every instance 

 of this kind, closer examination showed these examples to lie on the 

 edge or very close to joint planes or rifts in the strata through which 

 water had easy access. The beds of streams heading in the Knobs are 

 often crowded with geodes washed down from the higher strata. A 

 sketch of these conditions of occurrence would, therefore, be some- 

 thing like that shown on the preceding page. 



Having noted the occurrence of geodes, at least in these particular 

 strata, it is next in order to inquire into their method of formation. 

 To determine this point, the many specimens from the Knobstone 

 group are here employed, although examples from other geode- 

 bearing horizons, snch as the Upper Niagaran shales of West Tennes- 

 see, could be used. At this point it seems best for the present discus- 

 sion to insert some observations by the late Prof. N. S. Shaler. These 

 are incorporated in an article entitled Formation of Dikes and 

 Veins," from which title one would never suppose that a discussion 

 of geodes was included. The writer is indebted to Prof. J. C. Bran- 

 ner for calling attention to this article. 



Some light on the foregoing question [the formation of dikes and veins! ap- 

 pears to be afforded by observations which may be readily made on the formation 

 of geodes. As is well known, these bodies in their typical form are spheroidal 

 masses, usually of quartz, which are formed essentially in the manner of veins. 

 They may, indeed, lie termed globular deposits in this class; in fact, by extend- 

 ing the inquiry over a large Held I have been able to trace a tolerably complete 

 series of forms from spherical geodes to ordinary fissure veins, a series suffi- 

 ciently without breaks to warrant the assumption that all these bodies belong 

 in one category. A study of these geodes as they occur in Kentucky and else- 

 where, especially in the shales of the sub-Carboniferous rock, has afforded me 

 some interesting and instructive suggestions concerning the process of vein- 

 making which I will now briefly set forth. 



Normal geodes are hollow spheroids and are generally found in shales. 

 They clearly represent in most cases a segregation of silica, which has evidently 

 taken place under conditions of no very great heat, brought about by deep burial 

 beneath sediments or other sources of temperature. It is difficult in all cases to 

 observe the circumstances of their origin, but in certain instructive instances 

 this can he traced. It is there as follows: Where in a bed in which the con- 

 ditions have permitted the formation of geodes the calyx of a crinoid occurs, 

 the planes of junction of the several plates of which it is composed may become 

 the seat of vein-building. As the process advances these plates are pushed 

 apart and in course of time enwrapped by the silica until the original sphere 

 may attain many times its original diameter and all trace of its origin lost 

 to view, though it may he more or less clearly revealed by breaking the mass. 



a Bull. Geol. Soc. Ainer., X, 1899, pp. 253-2G2. 



