12th July.
The grass near the camp having been burnt off, the horses had scattered
very much, and could not be collected and saddled before 10.0 a.m., when
we followed our track of yesterday and reached the pool of water at
sunset. The country was so level, although we were crossing the watershed
between the north-west coast and the Gulf of Carpentaria, that the
aneroid only varied from 29.55 to 29.62, and even of this change the
greater part was caused by alteration of the temperature. The geological
character of this portion of the country differs slightly from that of
the Victoria River. The upper stratum is a bed of ironstone conglomerate
about twenty feet thick, this rests on sandstone, the upper part of which
is highly ferruginous, then passes away into a variegated sandstone
imperfectly stratified, changing into a hard siliceous sandstone which is
white and breaks with a conchoidal fracture; this rests on a hard cherty
sandstone similar to that of the Victoria River. In this rock many
depressions occur, which is apparently caused by the roofs of caverns
falling in and there are usually deep fissures in the rock at the bottom
of these hollows, in which all the water that drains into them is
absorbed; in some places the sandstone resting on the limestone has sunk
many feet below the general level, with areas varying from one to ten
acres, sometimes sloping towards a centre ten to thirty feet below the
plain, and in other spots with abrupt rocky banks five to eight feet high
and a perfectly level bottom. The level character of the country is
unfavourable for investigations of this nature, and the thickness of the
several strata not easily determined; but I think that the collective
thickness of the several strata above the limestone does not exceed 100
feet. The porous nature of the lower rocks preclude the existence of
permanent surface water by draining the whole of the upper part of the
tableland, while it forms strong springs in the lower ground towards the
banks of the Roper River, where the limestone is exposed on the surface.
Latitude by a Coronae Borealis 15 degrees 14 minutes 31 seconds.