F. 21. Van Tuyl — Geodes of the Keokuk Beds. 39 



" 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 heneath sediments 

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

 observe the circumstances of their origin, but in certain instruc- 

 tive 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 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 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 to clearly 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. Al- 

 though 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 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 wlien the temperature and pressure could not 

 have been great. It is readily seen that the walls of these hol- 

 low 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 have 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 sub- 

 jected as the conditions of its passage through the geode-bearing 

 zone are altered." 



More recently Bassler has written* on the formation of the 

 Knobstoue geodes. He says : 



"The majority of geodes in the Knobstone group may be 

 traced directly or indirectly to a crinoidal origin for the simple 

 reason that these strata are often crowded with the fragments of 

 this class of organisms. Probably next in order as a geode maker 

 is the common brachiopod Athyris lamellosa, but no class of 



* Proc. IT. S. Nat. Mas., vol. xxxv, p. 133 ff., 1908. 



