196 



Field Columbian Museum — Geology, Vol. i. 



In polarized light the yellow outer ring of the spherules remains 

 dark during a revolution, indicating it to be opal. The interior portion 

 assumes an appearance like that of granularquartz. This is similar to 

 that described by Hovey as characterizing the interior of some of the 

 spherules of the Pennsylvania oolite. The spherulitic interference 

 cross is, however, clearly to be seen, and the optical character is nega- 

 tive. Hence the method of formation can not be regarded as similar 

 to that proposed by Hovey for the siliceous oolite, but is probably a 

 simple spherulitic structure, in accordance with which any foreign 

 matter, such as the opal, was arranged. The main portion of the 

 chalcedony shows a typical radially fibrous structure 



I DRAWN WITH CAMKRA LUCIDA. 



Fig. 1. Section from " yolk " in ordinary 

 , 'light- Dark portion opal; light portion, in- 

 cluding spherules, chalcedony. 



X22. 



FlG. 2. Same in polarized light. The 

 opal becomes black; the chalcedony shows a 

 fibrous structure and the spherules interfer- 

 ence crosses. 



X22. 



A section of chalcedony from the veins of the region, studied for 

 comparison, showed a similar strongly marked fibrous character and 

 tendency to radial arrangement. 



The specific gravity of the specimen as a whole was found to be 

 2.594, or about that of chalcedony. 



It is of course desirable, if possible, to obtain an explanation of 

 the way in which so unique a case of petrifaction could occur, if 

 petrifaction it be. I have been unable to find any account of a simi- 

 lar occurrence as having been noted before. The fossil Chelonian 

 eggs of Tertiary age from Auvergne, France, are simply shells filled 

 with hardened mud. The fossil eggs of the New Zealand birds are 



