THUNDER-EGGS AND GEODES — BROWN 335 



sion, however, I am frankly unable to explain fully all the chemical 

 details attending their development. As no two specimens are exactly 

 alike, different chemical situations and reactions very likely prevailed 

 at different rates and times. Nevertheless, a generalized picture can 

 be drawn delineating the probable order of events. Consider, for ex- 

 ample, the section on plate 4, figure 1, which is similar to Renton's 

 unnumbered figure (1951, p. 175). The photograph was taken by 

 transmitted light, thus making the chocolate-brown outer matrix ap- 

 pear black. Within this is the irregular, squarish figure that outlines 

 a cross section of the original cavity, now filled with chalcedony, hor- 

 izontally banded below but unhanded above. A number of filaments 

 hang from the upper wall and bound areas of concentric ringed effects, 

 and some arise from the lower wall. Both the upper and lower fila- 

 ments penetrate the horizontal strata of chalcedony, as do also some 

 shown in plate 6, figure 1, passing through the left end of a thin white 

 band of opal. Further, it should be noted that a narrow, clear ( white) 

 zone makes a halo for the filaments and continues around the boundary 

 of the square but is not trespassed by the horizontal strata of the chal- 

 cedony. From these particulars it is possible to conclude that in this 

 specimen the sequence of events was roughly as follows: First came 

 the cavity, by one or another of the methods previously enumerated. 

 The cavity was surrounded by a chemically complex matrix that, 

 perhaps when still warm, underwent chemical changes as a concom- 

 itant of the circulation of connate as well as surface waters. These 

 waters, having picked up mineral reagents, including salts of iron and 

 manganese, and various silicates, passed through the porous walls or 

 seeped in through cracks, infiltrating and filling the cavity, the solu- 

 tion becoming a colloid or gel. According to gravitational and per- 

 haps electrical factors, such suspended, minute globules of mineral 

 matter as were present or were formed, arranged themselves below in 

 clear and dark stratified, horizontal layers, the upper part of the solu- 

 tion remaining relatively pure and not visibly layered. Then, as soon 

 as the chemical condition of the gel became suitable, there grew into it, 

 from the supply of salts along the walls, the filaments that are now 

 erroneously called algae. These pseudoalgae shot up rapidly in pulses 

 by chemical precipitation (probably as iron hydroxide and silicate), 

 osmosis, and diffusion. During this process, around each filament 

 there was generated a sharply defined field of chemical or electrical 

 influence that not only cleared the immediately surrounding gel but, 

 so to speak, held at arm's length at a fairly uniform distance any sus- 

 pended globules of mineral matter, thus creating around each filament 

 a transparent halo, the gel outside the halo remaining more or less 

 translucent and, where it contained dark mineral particles, becoming 

 concentrically zoned in the manner of Liesegang rings (Liesegang, 



