15© 



also, from other forms of silica by lower specific gravity and greater 

 solubility in solutions of potash. Its beautiful colours resulted from 

 rellection and refraction of light produced by peculiarities of structure. 

 There were several examples on the table, differing slightly from each 

 other in purity, &;c. A fine example of precious opal, in silu, from 

 the porphyry of Czernewitza in Hungary, would explain well how it 

 had been formed by infiltration in cavities much in the same manner 

 as agates. A box marked Tabasheer was interesting, as containing 

 specimens of this singular opal, formed in the interior of the stem of 

 the large Indian bamboo. 



And now that he had followed the struggles of silica, as it were, in 

 its way through the world, he had but little time left to devote to a 

 consideration of the splendour of its builded geometry, the beauty of 

 its crystalline rest. Various as the forms before them might appear 

 they were all derived from the six-sided prism terminated b)- a 

 hexagonal pyramid. This was the ideal form which, though it seldom 

 arrived at, it ever sought to gain . Seldom was the impurity so greatj the 

 obstruction so marring, impulse from v/ithin so wayward, or limitation 

 without so repressive, but we could trace some element of that more 

 perfect form to which, under happier circumstances, we knew it would 

 have attained. The opacity of some of the crystals was due to the air- 

 bubbles which had been entangled with them in their rapid crystalliza- 

 tion. The colour of many of the specimens was owing to oxide of 

 iron, of others to titanic oxide ; of the beautiful rose-quartz, to manga- 

 nese. The delicate purple of the amethysts was too evanescent for 

 satisfactory analysis. As interesting to him as the forni and colour 

 were the substances which we found enclosed in these crj'stals. Rutile, 

 mica, hornblende, and epidote were among the most frequent of these 

 enclosed substances. It was singular that while in chalcedony den- 

 dritic formations of peroxide of manganese were so often found, they 

 were rarely met with in rock crystals. They would look in vain among 

 that case of specimens for dendritic structures such as were 

 enclosed in the bits of limped and opaque chalcedony. In conclusion, 

 in writing this paper, he hardly hoped to add to the sum of human 

 knowledge ; but if he had succeeded in awakening an interest in a 

 subject, the study of which had been to him a source of infinite 

 pleasure, he should be more than satisfied. 



Throughout, Mr. Pankhurst illustrated his remarks by numerous 

 and valuable specimens. 



