66 T. W. E. DAVID. 
over the hard sandstones of the Hawkesbury Series, so that no 
undermining action is possible in the eastern area; hence the 
narrowness of the river gorges near the eastern escarpment. 
With reference to the date when the erosion of the present valleys 
of the Blue Mountains commenced, it must obviously have been 
later than that of the basaltic eruptions of Mounts Hay and King 
George, and later than that of the ancient river channel at Lap- 
stone Hill. The last two dates were perhaps late Mesozoic or early 
Tertiary. Whatever may be the date of the commencement of 
the formation of the fold, it is clear that that of the erosion of 
the valleys of the Blue Mountains must have nearly coincided 
with it. The depth of the valleys exceeds 1,500 feet. 
E. Relation of the Blue Mountains to the leitlinie of Australia. 
On Plate 4 are shewn the guiding lines of folding which have 
determined the principal orographic features of Australia. There 
is no evidence that in Pre-Cambrian, Cambrian, and Silurian time 
the eastern boundary of Australia approximated to its present 
outline. In Carboniferous time, however, an extensive folding 
took place (termed the ‘Gympie folding’ on Plate 4) which led to 
the development of the eastern cordillera of Australia, as else- 
where suggested by me.1 That the bulk of this folding was 
accomplished in Carboniferous time is proved by the fact that 
whereas the Carboniferous strata have been powerfully folded 
over wide areas, the Permo-Carboniferous rocks have been very 
little disturbed. The trend of the Blue Mountain fold approxi- 
mately follows that of the present coast line as well as of the 
continental shelf; and it therefore, perhaps, represents a renewal 
on a small scale in Mesozoic and Tertiary time of the folding so 
strongly marked in the Carboniferous strata of Australia. 
Summary.—There is evidence that a few miles to the west of 
the present western escarpment of the Blue Mountains, marine 
conditions obtained in Upper Silurian time, from at least as far 
south as the Monaro tableland to beyond the Jenolan Caves, 
Proce. Linn. Soc. N.S. Wales, Vol. vut., Series 2, Nov. 29, 1893, PP- 
582 - 597, and pls. 27 and 28. 
ee lage tek meals Lande See ee 
