89 
stony plains, and have already been referred to 
by Sturt as the stony desert. Owing to the exten- 
sive denudation of the desert sandstone the gibbers 
cover a considerable area of Central Australia. The lateral 
transportation of the stones by water action cannot be con- 
siderable, owing to the level contour of the intervening plains ; 
in fact, they are deposited, on the removal of the softer, un- 
derlying portions more or less vertically below their original 
position /// situ. On the slopes of many of the hills in process 
of disappearance the stony "wash" has accumulated in 
rounded terraces or steps, transported by torrential floods. 
The reflection of light from the smooth surfaces of these 
stones, when travelling towards the sun, is irritating to the 
eye. The glaze has been described by Mr. Brown as being 
"probably due to the action of siliceous water," and the effect 
is in small measiive increased by a slight, glossy surface coat- 
ing of precipitated iron oxide. The superficial polish has also 
been assisted, as has been suggested, by the action of wind- 
driven sand. 
The gibbers consist mainly of different varieties of quartz 
— forms of agate, jasper, chalcedony, and semi-opal — while 
in association with them occur concretionary forms of limon- 
ite, often assuming grotesque shapes. Gypsiferous clays were 
met with throughout the area covered by this formation, and, 
in them, large slabs of transparent gypsum that have been 
produced by crystalline intergrowth. In addition to these, 
various nodules, that occur in the softer portions of the rock 
and resist the denudation to a greater extent, are found. 
Ohsidicm Bombs (Volcanic). — These are widely distri- 
buted over the desert sandstone area, and have been the cause 
of much discussion, without any satisfactory deductions as to 
their origin. The phenomenon, which points to a former sur- 
face deposition, somewhere, of volcanic ejectanienta has given 
rise to various theories, such as meteoric, glacial, and of vol- 
canic action in sitii. Comparatively few examples were found 
during the Expedition, though single specimens were collected 
near to the Mann, Musgrave, and Ayers Ranges. I have, 
however, received a number of specimens from Mr. McXa- 
mara, from the neighbourhood of the Peake. Their univer- 
sal distribution has, no doubt, been assisted by the agency of 
the native and the emu (in the form of "gizzard stones"). The 
natives call obsidian bombs FandoUa and Kaleya korru, the 
latter meaning "emu eye." They are collected by the medi- 
cine men of the tribes, and applied in the healing of sickness. 
Recent Deposits. 
Sand. — With the exception of the various outcrops of 
rock previously discussed, sandy deposits cover all the ;^dia- 
