ARTESIAN WATER IN N. 8S. WALES AND QUEENSLAND. 409 
tions represented between the Atlas Mountains and the Sahara 
are Cretaceous, Miocene, Pliocene and Quarternary. With the 
exception of the Lower Cretaceous, none of the members of the 
Cretaceous system appear to be thoroughly pervious. The Cre- 
taceous rocks under a large area of the Sahara are covered under 
too great a depth of Post-Cretaceous deposits to be readily access- 
ible to artesian bores. The Miocene system, divided into the (1) 
Pecten numidus Beds (Lower) and Ostrea crassissima Beds (Upper) 
are water-bearing in places, and not too deep to he accessible, 
but by far the greater portion of the artesian water in the Sahara 
is obtained from the Pliocene Formation. 
The Pliocene strata of the Sahara are formed of white, grey, 
red, and yellow sand with clay beds and gypsum. The last is in 
places, as at Bard’Ad in the Oued Rir District, over eighty feet 
in thickness. Beds of hard limestone over a yard in thickness 
are met with occasionally. Immediately overlying the water- 
bearing sands and gravel is a very hard band of rock, a siliceous 
cement a foot or more in thickness. These Pliocene strata are of 
lacustrine origin, and lie in a broad basin, which through earth 
movements has veen thrown into a succession of folds. 
Mr. G. Rolland, in a paper read before the Geological Society 
of France, 14th of September 1885, entitled ‘‘ Ou et comment s’ 
alimentent les eaux artésiennes du bassin de |’ Oued Rir?” states 
that these strata derive their supplies of water partly from direct 
percolation of rain, partly from rivers, especially from those which 
take their rise in the Atlas Mountains on the north. Part of 
this water percolates into the permeable soil and finds its way into 
the deep alluvial deposits, which dip towards the interior of the. 
basin. The Pliocene strata are also partly supplied with water 
by springs rising from the Cretaceous strata, as for example the 
beautiful springs of Western and Central Zab and those of the 
Djerid. The massifs of the Cretaceous Formation attain an alti- 
tude in Aurés of over 7,500 feet, and the porous beds of the 
Cretaceous Formation which outcrop in these ranges are fed by 
the rain and melting snow on the higher ranges, and the under 
