X 
unaltered quartzite lieing extremely regular and clearly defined. The following 
section, illustrating the mode of occurrence of the deposits of })recious opal, 
was measured in the mines at White ClifPs : — 
I Rather loose conslonierates, consisting of pebbles varying from the size 
I of a jiea up to two feet or more in diameter. The pebbles are 
j- formed of Tapper Cretaceous (juartzib' (poreelainised rock), conglome 
I rate, ironstone, itc., anil the rock is considerably stained by ferric 
j oxide. 
I Very hard conglomerate, locally termed “geaser,” composed mostly of 
|> small pebbles of the size of a jiea, cemented by secondary silica. 
J This rock is considerably stained by ferric oxide. 
I Ifard, llinty, yellowish-white rock, passing gradually downwards into 
Soft white rock, of the character of kaolin, the joints stained in places 
by ferric oxide. Interspersed through this are occasional water- 
worn boulders (.sometimes two or three feet in diameter) of quartzite, 
containing characteristic Devonian fossils. The precious opal 
dejiosits appear to follow joints, cracks, and faults in the kaolin, 
occurring in horizontal seams as well as in vertical or highly 
inclined veinlets connecting the latter. These seams or veins of 
jirecious iqial are usually as thin as a knife, but open out occasionally 
to a thickness of an inch or more. 
Tlio following analyses of samples of the soft wdiite rock from different 
loealities Avere made hy Mr. II. P. White, Assistant Analyst to the Geological 
Survey : — 
1882. — Material from range 9 miles north-west 
of AVhite Clifts Opal Field. 
Per cent. 
Moisture at 100° C 5 40 
Combined water 5 01 
Silica (SiO'd 6812 
Alumina (At/ls) 15'25 
Ferric oxide (FeoO;,) '76 
Ferrous oxide (FeO) “27 
Manganous oxide (MnO) minute trace 
Copper oxide (CuO) absent 
Lime (CaO) '42 
Strontia (SrO) trace 
Magnesia (MgO) '77 
Per cent. 
Potash (K 5 O) '73 
Soda (NajO) '77 
Phosphoric acid (P 2 OJ) '01 
Vanadium pentoxidc (A^jOs) 03 
Titanic acid (TiOn) ’74 
Chromium .sesquioxide (Cr; 03 ) minute trace 
Sulphur trioxide (SO,) '10 
Chlorine (Cl) 1 -29 
100 -oo 
Less oxygen equivalent for chlorine '28 
100 -.38 
