BY ERNEST AV. SKEATS, D.SC, A.R.C.S., F.G.S. 21 



Professor Ewart kindly' examined the sections for me. 

 He found the material generally too fragmentary for pre- 

 cise determination, but noted the presence of sieve tubes, 

 cork cells, epidermis with cuticle, and in one place, part 

 of the woody tissue of a plant, probably a transverse sec- 

 tion of scahiriform tracheids. Some of the structures 

 appeared to represent sections of fresh water algae, others 

 of various plants, including the spore of a fungus, a trans- 

 verse section of a leaf and, possibly, a section of a small 

 petiole. Thei'e Avas a remarkable paucity of woody tissue 

 represented in the sections. 



In one of two places rounded or oval cellular areas 

 occur consisting of silica which now affects polarized light. 

 Their size and the character of their siliceous network 

 suggest that they may be altered radiolaria. 



Perhaps the most interesting organism is seen in Plate II 

 iig. 2. It consists of an oblique section through an append- 

 age of one of the arthropoda and may represent 

 the section of a leg of a fossil spider. The minute hairs 

 projecting from the surface of the appendage are clearly 

 noticeable in the photomicrograph. In New South Wales, 

 Victoria, and elsewhere, some deposits of common 

 opal are found to contain abundant skeletons of the siliceous 

 rfrustules of the diatomaceae, and close search for diatoms 

 has been made of the thin sections from this deposit, 

 but with entirely negative results. 



Mode of Formation of the Deposit. 



Keference to the sketch geological section, fig. 1, shows 

 that the black opal occurs in shalloAV depressions in the 

 surface of the basaltic flow which has been subsequently 

 covered with a younger flow. It would appear that in 

 the time interval between the two lava flows, erosion of 

 the earlier flow produced slight depressions which became 

 swampy. Plants growing near the depressed areas con- 

 tributed leaves to the deposit, while fresh Avater plants grew 

 and accumulated in the SAvampy depressions. The 

 mechanism by Avhich the material Avas coiiA^erted into opal, 

 cannot be clearly pictured, Ijut it is possible that thermal 

 waters stimulated by proximity to an active volcanic centrei, 



