1883. | The Geology of Central Australia. 1127 
oration that the region covered by such deposits are known collo- 
quially as “redstone plains.” The drift is often many feet in 
thickness, and contains silicified wood and casts of existing land 
and fresh-water shells. It is being, and has been, formed synchro- 
nously with the zolian sandstones, and in many places the latter 
merges indefinably into the stony drift. | 
Following the course of the many dry watercourses which in- 
tersect this region, are immense deposits of alluvium. Two dis- 
tinct formations can be recognized ; an older and much the larger, 
and a smaller recent one. The lower formation consists of beds 
of sand, gravel and clay. The stratification varies from the most 
regular deposits to beds showing oblique lamination and flow and 
plunge structure. These deposits extend some distance on each 
side of the watercourses, and vary greatly in thickness. It is in 
these beds that the most abundant remains of the extinct marsu- 
pials are found. They extend laterally far beyond the limits of 
the more recent deposits which overlie them. The latter consists 
of fine-grained, thin-laminated alluvium. It contains no remains 
of Dipr otodon, &c., but existing fresh-water shells such as Ano- 
don, Paludina, Physa, &c., are abundant. It is formed by the dep- 
osition of sediment which the periodic floods bring down. 
Such, as far as is now known, is the structure of Central Aus- 
tralia. 
At the close of the Archean age the rocks which we have de- 
Scribed as belonging to that age, were crystallized, folded and 
raised above the surface of the primeval sea, forming islands 
which probably have not since been completely submerged. 
After this elevation followed an immense period of quietude, 
during which vast beds of sediment derived from the erosion of 
these rocks, were deposited in a Paleozoic sea. Then at the end 
of the Cambrian or Silurian period another elevation occurred, 
and the rocks forming the Flinder’s range were crystallized and 
upheaved. As I have said, at the northern extremity of this 
formation the Mesozoic rocks lie unconformably on it; so that 
there is a complete gap between. It is probable, however, that 
the Mesozoic beds are underlaid by Devonian and Carboniferous 
deposits. The structure of the eastern border region lends prob- 
ability to this view. At any rate the whole lapse of time between 
the upheaval of the Cambrian or Silurian rocks until the close of 
Jurassic, must have been, in the interior basin, a period of 
