20 PLANT-BEARING COMMON BLACK OPAL, . 



■was left uncovered, so that the refractive index i)f tlie 

 material could be determined against various oils. 



The refractive index of the material was found to he 

 very low, lower indeed than that of any known rock. 



It was found to have almost the same refractive index 

 as that of chloroform, namely 1.45. The refractive index 

 of tachylyte was found to be considerably above 1.53. 

 In thin sections the material is a brownish-red colour, 

 contains, in places, rounded or irregular concretions of 

 pyrite, and is completely isotropic except for minute grains 

 of quartz and possibly felspar embedded in it. In contains 

 no microlites. 



Chemical Characters. 

 Blowpipe tests showed the abundance of silica by the 

 residue or skeleton left in a bead of microcosmic salt : 

 in the closed tube much water was condensed on heating, 

 strong reactions of sulphur were obtained on a silver coin 

 and the residue, after heating, being magnetic, indicated 

 the presence of iron. It will be noticed that all the physical 

 characters, such as specific gravity, refractive index and 

 isotropism, as well as the chemical characters, agree with 

 those of the mineral common opal. The colour and lustre 

 of the material are. however, unusual. It is dense, dull 

 black in colour, with a pitch-like lustre and a notable 

 conchoidal fracture. The occurrence of irregular masses 

 of nodules of common opal, associated with volcanic rocks, 

 such as trachytes and basalts is, of course, not uncommon, 

 but such as I have seen have been either creamy white. 

 3'^ellow, 1)rown. or greenish in colour. The black opal 

 is interesting also from the fact that it is crowded ^^^th frag- 

 ments of fossil plants. 



FOSSILIFEROUS CHARACTERS. 



.Several thin sections of the black opal were made 

 and examined. It was at once noticed that these sections 

 were largeh composed of the remains of plants or rather cell 

 structure of vegetable origin The plant material was very 

 much broken up and disintegrated as a reference to the 

 photomicrographs, Plate I, figs I and 2, and Plate II., 

 fig. 1. will show. 



