THUNDER-EGGS AND GEODES — BROWN 339 



EXPLANATION OF PLATES 

 Plate 1 



1. Section of a thunder-egg from 16 miles northwest of Del Norte, Colo. The elliptic, 

 original nucleus lies below the triangle. Inside, a dark rim of chalcedony encloses a white 

 patch of quartz. 



2. Perlite from 5 miles south of Buchanan, Oreg., containing a number of thunder-eggs. 

 The original spherulitic nuclei were disrupted and sheared before being filled with chalced- 

 ony (white). Collected by French Morgan. Both figures natural size. 



Plate 2 



1, 2. Thunder-eggs from the Priday (Fulton) ranch near Antelope, Oreg. The angular 

 figures are surrounded by a chocolate-brown matrix. Inside they are opal, chalcedony, and 

 quartz. Natural size. Figure 2 collected by C. H. Robinson, Sr. 



Plate 3 



1, 2, 3. From the Priday (Fulton) ranch near Antelope, Oreg. 1 and 2 show chalcedony 

 occupying cavities that were eccentric to the spherulitic nuclei (eyes). If the brown matrix 

 were removed from figure 2 the remaining chalcedony would look somewhat like that in 

 figure 3, with a ball on the top side. 



4. Geode from 25 miles southwest of Blythe, Calif. The center is studded with quartz 

 crystals. Below, the layers of chalcedony lie at different angles to one another and indicate 

 that the lava enclosing the original cavity was tilted at least twice. All figures natural size. 



Plate 4 



1, 2. From the Priday (Fulton) ranch near Antelope, Oreg. The photograph of the sec- 

 tion shown in figure 1 was taken by transmitted light. Pseudoalgae from above and below 

 penetrate the layered chalcedony. Continuous around the wall and the filaments is a halo 

 of transparent chalcedony (white). Liesegang rings appear between the filaments in the 

 upper part of the figure. Natural size. 



3. From Hog Creek, 9 miles northwest of Weiser, Idaho. This section shows that after 

 the pseudoalgae had formed, portions of their substratum loosened and floated in the gel. 

 Natural size. Collected by French Morgan. 



4. Righthand part of figure 3, magnified 10 times.f^The filaments look like small intes- 

 tines. 



Plate 5 



1, 2. Filaments and plumes in thunder-eggs from Priday (Fulton) ranch near Antelope, 

 Oreg. Natural size. 



3. From a lava flow 17 miles south of Alpine, Tex. Only a color photograph could do 

 justice to this section. It shows an intricate network of minute filaments that developed 

 from several centers as indicated by the arched curves. A Natural size. Collected by Frank 

 L. Hess. 



4. Lefthand part of figure 3, magnified 10 times. It shows the irregularity in thickness 

 and branching of the pseudoalgae. 



Plate 6 



1, 3, 4. From the Priday (Fulton) ranch near Antelope, Oreg. Figure 1 shows the pseudo- 

 algae in cobweblike sheets and fringes. Some filaments pass through the lefthand end of 

 the opal (white) band, evidently having grown when the band was still a gel. Figures 1 

 and 4, natural size; figure 3 magnified 10 times. 



2. Locality doubtful, but from the color of the filaments and general appearance of the 

 matrix, apparently Hog Creek, 9 miles northwest of Weiser, Idaho. Natural size. 



