AN EXAMPLE OF DISRUPTION OF ROCK BY LIGHTNING 

 ON ONE OF THE LUCITE HILLS IN WYOMING^ 



V. H. BARNETT 



The accompanying picture (Fig. i) is a view on the summit of 

 Cross Mesa,^ one of the Lucite Hills near Rock Springs, Wyoming. 



This mesa, like most of the group, is quite barren and flat on 

 top, the volcanic rock of which it is composed being unprotected by 

 soil and vegetation. Like the other Lucite Hills it is a very prominent 

 landmark standing well above the surrounding country. 



The angular bowlders seen in the picture have been torn by some 

 apparently violent force from the surface of the lava and some of them 

 still lie in the cavity formed. The space from which the rock frag- 

 ments were torn is roughly a half-saucer in shape, having the east 

 rim nearly vertical while on the opposite side it is more gently sloping. 

 Two or three cracks, one of which may be observed near the right 

 lower corner of the picture, radiate from the saucer-shaped depression. 

 Whether some of these cracks may not have occurred before the dis- 

 ruption the writer was not able to judge, but it is not likely that all 

 of them did so occur. The rock fragments range in size from an inch 

 or two in diameter up to about two feet and a half, and are sharply 

 angular with fresh surfaces. From the size of the cavity the amount 

 of rock removed is approximately twelve cubic feet and lies within 

 a radius of about ten feet from the fracture and exclusively to the 

 west of it. No fragments were observed to have been thrown very 

 far. 



Two hypotheses at once present themselves in explanation of this 

 phenomenon: first, that of an artificial explosion as dynamite or 

 blasting powder; second, that of lightning. 



The probabilities of this being due to the first hypothesis seem 

 very slight since it is so far removed from human activities of any 



1 Published by permission of the Director of the U. S. Geological Survey, 



2 J. F. Kemp and W. C. Knight, "Lucite Hills of Wyoming," Bull. Geol. Soc. 

 Amer., Vol. XIV, 1902, p, 317. 



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