70 
DtontTB 
Fig. 4. — Step-faulted Gneiss and Diorite Dyke, North-east 
Mann Ranges. 
An interesting phenomenon was encountered in thi* 
series of gneisses some dozen miles north-west of the western 
extremity of the main range, where low outcrops skirt the 
eastern limit of a large depression or ''salt pan," the saline 
deposits of which rest directly upon a bed of similar gneissic 
rock. These outcrops have weathered by a process of ceolian 
erosion into mushroom-shaped masses (PilzfelsenJ, with 
smooth central columns, narrow at the base, and gradually 
widening upwards to support a flat, tabular mass at the top. 
The stalk is abraded by deflation, the wind hurling tha 
coarse grains of sand, which do not rise to beyond a few feet 
above the level of the ground, incessantly against the base 
of the column. (Plate xiii., fig. 2.) 
Streich has reported* mushroom-like forms of sand ta 
occur in the wind-drifted sands of the Great Victoria Desert. 
He states that the sand is generally loose, though somewhat 
consolidated by means of a clay cement, but only on the 
surface. When the uppermost crust has been broken 
through, the wind gradually blows away the underlying 
loose sand, leaving the upper layer unsupported around the 
* Scient. Res. Elder Expl. Exped., 1891-2, Geology. 
Roy Soc. S.A,, vol. xvi., page 88. 
Trans. 
