Plantlike Features in Thunder-Eggs 

 and Geodes 1 



By Roland W. Brown 



U. S. Geological Survey 



[With 6 plates] 



Long ago, according to legend, the angry gods on Mount Hood and 

 Mount Jefferson in the scenic Cascade Range of Oregon fought a 

 titanic battle during a violent storm. Amid flashes of lightning and 

 peals of thunder the opposing artillerists hurled thousands of round 

 rocks at one another across the intervening 60 miles. Many of these 

 thunderstones fell, far wide of the intended targets, over a large area 

 to the east of the mountains. The embattled gods presumably obtained 

 these missiles by robbing the nests of thunderbirds. Consequently, 

 when retrieved today they are popularly called thunder-eggs (Renton, 

 1951, p. 172), although they are not egg-shaped but roughly spherical 

 and quite variable in size. Vivid myth aside, the plain fact is that 

 these thunder-eggs, or spherulitic geodes as they are known among 

 geologists, are restricted to the weathered outcrops of a prehistoric 

 (probably late Oligocene or early Miocene) lava flow, which is now 

 a rhyolitic, welded tuff in whose glassy matrix they originated, with- 

 out benefit of thunderbirds, as everyone familiar with them in the field 

 knows. Moreover, closely similar specimens occur under somewhat 

 comparable circumstances at numerous other localities not only in 

 Oregon but in distant parts of the world. 



The occurrence, appearance, and origin of thunder-eggs have been 

 discussed more or less adequately by amateur rockhounds and pro- 

 fessional penologists (Dake et al., 1938; Renton, 1951; Koss, 1941). 

 Therefore, I shall here refrain from unnecessary repetition of com- 

 monly known details but shall deal particularly with a pseudobotani- 

 cal phase that, at least among collectors of gem materials, has aroused 



Publication authorized by the Director, U. S. Geological Survey. Grateful 

 acknowledgment of help through discussion is hereby accorded to Robert L. 

 Smith, U. S. Geological Survey ; E. P. Henderson, U. S. National Museum : and 

 French Morgan, 2601 Brentwood Road, NE., Washington, D. C. 



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