258 J. MILNE CURRAN. 
with cold hydrochloric acid. The specimen now exhibited con- 
taining some noble opal, is a typical white and hard marl. In 
places, as might be supposed, this marl is deprived of the carbonate 
of lime and then approaches a kaolin, while it is in most cases 
stained with iron oxides. The opal now mined is won entirely 
from this marl-stone, which probably does not go down more than 
some sixty feet. Below this there is eighty feet of a soft and clayey 
rock, locally called “slum,” a bed not at all likely to contain opal. 
Under the so called “slum” is a bed of sandstone into which a 
well was put to the depth of twenty-five feet without finding any 
traces of opaline rock. About nine miles to the north-east of 
White Cliffs, the horizon on which opal is found is seen to be 
succeeded by yellow ochre-like clays. White claystones rest on 
these and the series is capped by a siliceous conglomerate. 
Origin of the Opal. 
*One of the first facts to be noted on the opal field is the associ- 
ation of the opal with lenticular masses of a rock, known to the 
miners as “angle stone,” and the “guardian angle stone.” This 
rock has all the appearance of a diatomaceous earth. The rock 
is also known as “biscuit stone,” and this expresses well the 
peculiar and well known feel of indurated diatomaceous earths. 
I have made a number of slices of this rock, but have not been 
able so far to recognise diatoms or radiolaria.!_ I may state that 
the “angel stone” lies in layers above the opal, and in my opinion 
is a diatomaceous deposit, or certainly one of organic origin, and 
that it furnished the silica to form the opal. 
*In this connection I may say that I have more than once 
received specimens of diatomite from the Richmond River, which 
were in part converted into a true opal. Mr. Krause, Lecturer 
on Mining at the Melbourne University, describes’ a tripolite 
deposit from Lilicur (Vic.). The deposit is seventeen and a half 
feet in thickness, and covers an area of four and a half acres. He 
1 I have detected certain spherical bodies, but having no specia ial k 
ledge of these oe must hesitate to ‘decide whether they sls oF 
getgh not be radiolaria. 
2 Royal Society, Victoria, Vol. xx111., p. 250. 
