ILLUSTRATION OF VEIN-FORMING FROM GEODES 261 



about b}'- deep l)uri;il beneatb sediments or other sources of temperature. 

 It is difficult ill all cases to observe the circumstances of their origin, but 

 in certain instructive instances this can be traced. It is there as follows : 

 Where in a bed in which the conditions have permitted the formation 

 of geodes the calyx of a crinoid occurs, the planes of junction of the sev- 

 eral plates of Avhich it is composed ma}^ 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 be more or less clearly revealed by breaking the mass. 



In the process of enlargement which the geodes undergo they evidently 

 provide the space for their storage by compressing the rock in which they 

 are formed. In the rare instances where I have been able clearly to ob- 

 serve them in their original [)osition they were evidently cramped against 

 the country rock, the layers of which they had condensed and more or 

 less deformed. Although when found upon the talus slopes or the soil 

 these spheres usually contain no water in their central cavities, these 

 spaces are filled with the fluid while they are forming and so long as 

 they are deeply buried. There can bo no doubt that this water is under 

 a considerable though variable pressure. 



The conditions of formation of spheroidal veins or geodes clearly indi- 

 cate that an apparently solid mass of crystalline structure may be in 

 effect easily permeated by vein-building waters, and this when the tem- 

 peratures and pressures could not have been great. It is readily seen 

 that the walls of these hollow spheres grow interstitially while at the same 

 time the crystals projecting from the inner side of the shell grow toward 

 the center. We therefore have to recognize the fact that the silex-bear- 

 ing water penetrated through the dense wall. In many of these spherical 

 veins we may note that the process of growth in the interior of the spheres 

 has been from time to time interrupted and again resumed. These changes 

 may be due to the variations in pressure to which the water in the cavities 

 is necessarily subjected as the conditions of its passage through the geode- 

 bearing zone are altered. 



The most important information we obtain from the stud}^ of spherical 

 veins or geodes is that no distinct fissures or rifts are required for the pas- 

 sage of vein-building waters through existing masses of lodes. It is true 

 that the distances they traverse in these spherical lodes is limited to, at 

 most, a few inches ; but there is in these cases no other impulse than 

 diffusive action to bring about the movement, while in an ordinary tab- 

 ulate vein we may generally assume, in addition to the infiuence oi)er- 

 ating in bringing the dissolved materials into thegeode, a pressure which 

 impels the fluid upward. Thus, while it is not to be denied that many 



XXXVIII— Bull. Geol. Soc. Am., Vol. 10, 1898 



