138 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxxv. 



In the process of enlargement which the geodes undergo they evidently pro- 

 vide 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 observe them 

 in their original position 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 rilled with the fluid while they 

 are forming and so long as they are deeply buried. There can be no doubt that 

 this water is under a considerable though variable pressure. 



The conditions of formation of spheroidal veins or geodes clearly indicate 

 that an apparently solid mass of crystalline structure may be in effect easily 

 permeated by vein-building waters, and this when the temperatures and pres- 

 sures 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-bearing 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 study of spherical veins 

 or geodes is that no distinct Assures or rifts are required for the passage 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 tabulate vein we may generally 

 assume, in addition to the influence operating in bringing the dissolved materials 

 into the geode, a pressure which impels the fluid upward. Thus, while it is not 

 to be denied that many veins are prepared for by the formation of somewhat 

 gaping Assures, and that these rents, after being more or less completely closed, 

 are reopened by faulting on the plane of the deposit, such original or secondary 

 spaces are not required for the development of a vein. The other point is that 

 the pressure of the growing vein, which in the case of the geode is able so to 

 condense the rock matter about it as to win room for the deposit, is likely to 

 be even more effective in the group of tabulate deposits in forcing the walls 

 asunder. 



While the purpose of Professor Shaler's remarks is primarily to 

 show the relationship of geodes to veins, he has also pointed out some 

 factors in the formation of the former. The writer must object to 

 the following of his conclusions : First, that geodes are formed when 

 deeply buried; second, that the water of formation is under a con- 

 siderable though variable pressure; third, that the geodes are appar- 

 ently solid masses of crystalline material, which may in effect be easily 

 permeated by vein-building waters; fourth, that the silex bearing 

 water necessarily penetrated through the dense wall; and fifth, that 

 no distinct fissures or rifts are required for the passage of vein-build- 

 ing waters in the formation. Professor Shaler's statements upon the 

 formation of these objects were based upon a study of specimens from 

 the same horizon as those used by the writer, so that both sets of speci- 

 mens must have had a common origin. The geodes in these Subcar- 

 boniferous. or Knobstone strata, are invariably siliceous and appar- 



