XI 
Permo-Carboniferous System. These islands belong to the Rhacopteris Stage 
of the Carboniferous System, and are composed largely of rhyolitic lavas and 
fine tuffs and tuffaceous shales, together with later flows of plagioclase 
trachyte and hypersthene-andesite. Mantling around these old shore lines 
are coarse and massive conglomerates in which have survived only strong 
and thick shells such as Eurydesma cordata and Platyschisma oculum. 
The horizon of these coarse conglomerates is about 200-800 feet 
below that of the foraminiferal limestones at Pokolbin. The conglomerates 
are partly tuffaceous, the tuff being of an andesitic character. At Pokolbin 
they form the base of the Permo-Carboniferous System, though they are by 
no means the lowest strata of that System as represented in the adjoining 
districts. Por example, at Lochinvar, about eleven miles north-easterly 
from Pokolbin, over 2,500 feet of Permo-Carboniferous strata have been 
developed below the horizon of the conglomerates. The disappearance of 
these lowest strata between Lochinvar and Pokolbin is due to a gradual 
transgression of the conglomerates in the direction of Pokolbin on to the 
pre-Permo-Carboniferous inliers of Mt. Bright and its associated hills. In 
places a sheet of dolerite with red olivines and containing an abundance of 
secondary natrolite with datolite and analcite 1 overlies the conglomerate and 
underlies the foraminiferal limestone. The volcanic particles in the lime- 
stone appear, however, to have been derived from the older (andesitic) 
eruptions rather than from these later basic lavas. 
As the material of the rocks above the conglomerates becomes finer- 
grained upwards, passing into mudstones and sandy shales, until the horizon 
of the foraminiferal limestone is reached, it is probable that the limestone was 
developed under conditions of subsidence, the adjacent volcanic islands of the 
Carboniferous System being more or less rapidly submerged, until eventually 
Poraminiferal limestone was formed. 
( b ) As regards the nature of strata separating the tiro foraminiferal 
horizons, a thickness of about 200 feet of sandy shale was deposited above the 
limestone with occasional thin ostracod limestones. The horizon of the 
Ravensfield Sandstone follows next in ascending order. This marks an 
epoch when the subsidence became slower, about 800-1,000 feet of the 
sandstones, known as the Parley Beds (Parley Stage), being then laid down. 
The former existence of floating ice in the sea of the Parley Stage in this 
district is proved by the presence of occasional erratics. At the close of the 
1 C. Anderson, M.A., B.Sc., Records Austr. Mus., 1904, V, Pt. 2, pp. 127-130. 
C 
