602 
MOONDILLA GOLD FIELD. 
Although widely different in its physical aspects, this Goldfield belongs, I am 
fully persuaded, to the same category as Mount Morgan, representing another phase of 
the activity of thermal waters, probably manifested at or about the same period of time. 
The “ Goldfield” occurs on the dividing line between Gumbardo and Milo Euns, 
in the middle of nearly dead-flat mulga-covered pastoral land. What may be called the 
bed-rock of the district is the “ Eolling Downs” or *’ Dower Cretaceous” str.ata, which 
have already been described. This is covered by the material about to be described, 
and which can be identified as derived from the dehris of the Upper Cretaceous or 
Desert Sandstone, of which denuded fragments occur in the neighbourhood. 
The groundwork of the field consists of what is known as “ cement,” a some- 
what consolidated “ wash of materials for the most part siliceous, but to some 
extent felspathic. This cement, on being broken up and washed, is found to contain a 
considerable quantity of hydrous silica, which is not in rounded waterworn grains, but 
in concretions or nodules. Sometimes the silica, when present in large quantities, forms 
lenticular concretionary masses, and even what may be called beds, strata or seams 
interstratified with the cement. When these beds attain any thickness they present 
almost every variety of the chalcedony group of hydrous silica, varying in colour from 
smoky to yellow, pearly, milk-white, and water-clear, and, while breaking with great 
facility (with a conchoidal fracture) into “ knives ” or “ flint-flakes,” can be seen on 
every surface and in every thin section to have a nucleated structure throughout. 
The above characteristics, and above all its low specific gravity, would enable any 
miner to distinguish this from the auriferous stone of any hitherto known goldfield. 
The low specific gravity is, of course, due to the fact that water enters into its 
composition, so much of it as is water being only two-fifths of the weight of an equal 
hulk of common anhydrous quartz. 
If it be permissible mentally to reconstruct the conditions under which the 
Goldfield was created, we may imagine, in the first place, a land surface on which the 
Upper ^Cretaceous (Desert Sandstone) lay on the Lower Cretaceous (or “Eolling 
Downs ”) strata. In course of time the Desert Sandstone was denuded till mere 
fragmentary tablelands of it remained in situ, the debris of the portion removed being 
spread over the surrounding low country. Prom abundant evidence furnished by other 
parts of Australia, it is known that at a period (Newer Tertiary ?) when the denudation 
of the Desert Sandstone had gone on for some time, immense outbursts of volcanic 
activity took place, giving rise in some cases to flows of ba.«altic lava, and in others to 
fountains of warm water. Wo may presume that the latter was the case at Moondilla, 
and that the water was highly charged with dissolved silica, of which it carried quantities 
into the then loose and incoherent masses of debris formed of the waste of the Desert 
Sandstone. The silica, at first in a gelatinous condition, would eventually he segregated 
into concretions surrounding nuclei or filling up spaces or openings in the bedding- 
planes, and on consolidating and crystallising would have no difficulty in elbowing 
portions of the cement matrix out of the way and assuming the gnarled and twisted 
forms presented by the majority of the concretions and seams. In aU probability the 
thicker beds of silica represent pools of silica-charged water, which were ultimately 
covered up by further accessions from the debris of the Desert Sandstone. With the 
silica a limited amount of gold appears to have come up in solution, and to have been 
deposited with it, possibly precipitated by the agency of sulphate of iron. The portions 
of tlie silica richest in gold are those whicli are seamed with iron oxide. 
The silica occurring in the cement has evidently had a similar origin to that which 
forms comparatively thick beds. It is not waterworn, and has not been carried by 
