81 



NOTES ON SOME INCLUSIONS OBSERVED IN A 



SPECIMEN OF QUEENSLAND OPAL. 



By D. A. Porter, Esq., Tamworth. 



[Read before the Royal Society ofN.S.W., 3 August, 1887.'] 



Recently while examining a specimen of " Opal-matrix " from 

 Queensland (exact locality not known) I noticed several included 

 vegetable remains, apparently portions of the stems of aquatic 

 plants. As there is no record, so far as I am aware, of such 

 having been before observed in Opal, perhaps a few observations, 

 on the specimen abo\ 7 e referred to may not be without interest. 

 This stony material (matrix) is of a rich chestnut-brown colour, 

 and quite opaque, except where cavities have existed and have 

 since become filled with Opal. These portions are translucent 

 and of rich purple, blue and green colours. Many of the opal 

 segregations are cylindrical in form, and penetrate the specimen 

 in various directions. Cross sections of these cylindric deposits 

 show that the places now containing the opal, were (in some 

 instances at least) originally formed by embedded stems or other 

 portions of plants. In many cases portions of the vegetable tissue 

 remain, completely incased in and preserved by the silicious matter. 

 The structure of the air cells in the remains is easily seen with 

 an ordinary good pocket magnifier. The accompanying figures 1,, 

 2, 3, 4, are good representations of four of the inclusions observed. 

 The figures are however very much enlarged, Nos. 1 and 2 about 

 10 diameters, and 3 and 4 about 40 diameters. I made an 

 attempt to cut a section, but failed through the brittleness of the- 

 stone, and my want of experience in manipulation. 



Should it be found that the opal bearing rocks of Queensland 

 contain any large quantities of the remains of plants, it would 

 seem to indicate the probability of the opal silica — in that 

 particular locality at least — having been gathered from the soil in 

 the first instance by plant life. Silica in the hydrated form being, 

 as we know, taken up in no small quantities (comparatively) by 

 certain plants, and in some instances in greater quantities than 

 the plant is capable of using up ; as an instance I may mention 

 the " Tabasheer " of the Bamboo, a jelly-like substance, which is 

 found in the joints of some species of that plant, and which is 

 composed of over 80 per cent, of hydrous silica. Our coal beds 

 also give upon analysis a mean return of about 3 per cent, for 

 silica contained therein. Organic silica is yet again found in 

 considerable quantities in several places, as diatomaceous earth. 

 Thus to me it does not seem improbable that in many instances, 

 the opals may have been formed by the solution and re-sol idification. 

 of the skeletons of diatoms and of the silicious coatings and. 

 segregations of plants higher in the scale than the diatomacese. 



F- August 3, 1887. 



