336 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1956 



1907). Here, also, the concentricity of the rings was influenced in 

 some way by the presence of the filaments, for the curves of the rings 

 parallel those of the filaments faithfully. It seems clear, therefore, 

 that the filaments originated after and not before the cavity was filled 

 with the gel. 



How long the gel in this thunder-egg remained in that state is 

 problematical, but eventually some as yet unknown condition initiated 

 the transformation of the soft gel into cryptocrystalline agate and 

 chalcedony, thus embedding the fragile filaments in a hard matrix and 

 preventing their destruction by earth movements. A binocular micro- 

 scope reveals that the fibroid crystals of the chalcedony are now 

 arranged radially with respect to the filaments as centers. Apparently 

 not much happened internally to this specimen subsequent to the events 

 just outlined. Externally, it remained a roughly spherical, ridged 

 object in the enclosing tuff until it weathered out as a familiar 

 thunder-egg. 



Allowing for variations, the geological story of all thunder-eggs is 

 similar to that just given. Some of the variations need description 

 because they supply, as it were, a supplement to the story. Consider, 

 for example, the section shown in plate 1, figure 1. Here the central 

 white part is solid quartz enclosed by dark chalcedony. In plate 2, 

 figure 1, and plate 3, figure 4, the central parts have unfilled cavities 

 attractively studded with quartz crystals. In these instances probably 

 some change in alkalinity stopped the formation of chalcedony and 

 started the crystallization of quartz, which proceeded until all the 

 remaining silica was used up. This final step may leave a crystal- 

 lined geode filled with water ; but there may be no water left because 

 fracturing subsequent to the completion of the geode permitted it to 

 escape. In plate 3, figure 4, the divergent angle between the layers of 

 chalcedony indicates that the rock mass containing the geode was tilted 

 after the first layering ended. In some plume agates, notably those 

 in possession of A. W. Hancock, Portland, Oreg., the plumes occur in a 

 series of conditions from complete burial in chalcedony to perfect 

 exposure in partly filled cavities. From these examples one might get 

 the casual impression that the plumes developed in the manner of 

 helictites in caves and that the silica came in later but did not embed 

 all of the plumes. Such appearances, in my opinion, are deceptive. 

 The more probable likelihood is that the cavities were originally filled 

 with a gel solution and that the plumes grew normally in it but that 

 the gel became too dilute or leaked out before it could be transformed 

 into chalcedony, thus leaving some plumes partially or wholly exposed. 



This explanation of the pseudoalgae in thunder-eggs may, in part, 

 also apply to some of the "mosses" in the well-known moss agates. 

 No one, to my knowledge, however, has seriously claimed that these 

 "mosses" represent former living organisms. 



