THTJNDER-EGGS AND GEODES — BROWN 333 



gases as the cause, some the relief of hydrostatic pressure and tension, 

 and still others the recrystallization of the original material of the 

 spherulite followed by formation of a "mud," desiccation, and shrink- 

 age. All apparently agree that in general the evolution is from a 

 solid spherulite (see Renton, 1951, fig. 2, p. 172) to a lithophysa or 

 rock bubble (see Stearns and Isotoff, 1956, pi. 2, fig. 3), sometimes 

 sheared (pi. 1, fig. 2), and finally the filling of the cavity with silica 

 minerals. The process may not have followed a universal rule in each 

 instance but may have differed considerably according to accompany- 

 ing physical and chemical circumstances. Thus, in many thunder-eggs 

 the original spherulitic center remains intact as a small nucleus or 

 ball and the cavity lies to one side of it (pi. 3, figs. 1, 2) . In others the 

 nucleus was obviously disrupted, the separated segments still showing 

 the original radial arrangement of the crystalline matrix (pi. 1, fig. 2). 

 The best analogy, but not necessarily explanation, for the resulting 

 shape of the cavity, is by Iddings (1888, p. 263) : " . . . the central 

 mass of the more open [ones] appears to have shrunken and cracked 

 apart like the heart of an overripe watermelon." From these dif- 

 fering views one may draw the conclusion that perhaps before a final 

 statement can be made more field observations and laboratory work 

 are needed to check or imitate the natural effects. 



As to how the filaments themselves originated and became embedded 

 in chalcedony at least two known phenomena point to explanations. 

 These are the natural helictites sometimes seen in caves and other 

 sites, and the so-called chemical gardens grown in the laboratory. 



Natural limy helictites (not stalactites or stalagmites) are moder- 

 ately small, irregular, twisted, tubular structures that develop from 

 cave ceilings, walls, and sometimes stalactites. They grow at their 

 free ends, being fed by internal solutions flowing distally from the 

 points of attachment. Thus, so far as method of growth is concerned, 

 they are not quite the same kind of thing as salt and ice ribbons 

 (Brown, 1946), which grow from their bases. Natural helictites can 

 be imitated artificially with simple, chemical apparatus (Huff, 1940). 



Did the filaments in thunder-egsrs arise like natural helictites by 

 growing out from the walls into empty cavities ? Renton ( 1951, legend 

 to unnumbered figure, p. 175) virtually concluded that they did, but 

 he overlooked the threads subtly concealed within the "stalacites" he 

 described as hanging from the roof of the original cavity and inferred 

 were later embedded in agate that filled the cavity from below. Yet, 

 on the following page he reversed himself and said : "The writer be- 

 lieves that most of the 'moss' seen in thunder-egg agate entered the 

 original cavity from below and the mineral bearing solutions [that 

 became 'moss'] were injected into a silica gel filled cavity ..." 



Here the unknown intrudes itself. Assuming an empty cavity to 

 begin with, how long after its formation did it remain empty, except 



