Journals of Australian Explorations

by Augustus Charles Gregory; Francis Thomas Gregory

GRANITE, PORPHYRY, AND SLATE.

28th September (Sunday).

Walked out from the camp to a low hill about one mile south-south-east. It was composed of granite at the base and capped with horizontal strata of sandstone, some of the beds containing large water-worn pebbles, and the superstratum highly ferruginous. To the south-west of this hill the rock was slate, the strata nearly vertical; the strike north and south, but much contorted, and large pebbles of porphyry, quartz, slate, granite, sandstone, and agate formed banks in the bed of the river. The country, as seen from the hill, was generally level in appearance, but consisted of numerous low ridges and detached hills of granite, with sandstone on the summits. The valley of the river extended to the east and south, and a large branch appeared to join from the south about ten miles lower down, as a valley and some ranges of hills trended in that direction. The whole face of the country had an arid and desolate aspect, as there were no large trees except along the principal watercourses, and many of the hills appeared destitute of any other vegetation besides small acacias and scrub trees, the bare rock showing through its scanty covering.

29th September.

At 7.15 a.m. again steered east up the river, the country level and timbered with stringybark, box, bloodwood, leguminous ironbark, and rusty gum; the soil a red sandy loam, thinly grassed; at 10.30 a.m. came to low hills and ridges of granite and porphyry, timbered with box, leguminous ironbark, terminalia, and the grass somewhat improved. Altered the course at 11.0 a.m. to the south to close in with the river; but after crossing a great number of dry watercourses, and even steering west, only reached the bank of the river at sunset. The channel was dry, and all the vegetation had disappeared, only a barren waste of coarse sand and gravel 180 yards wide, with bare rocky banks, showed that it had once been a running stream. A few small hollows in the rocks retained water from the late rain, but not sufficient for our horses, and though we found a small pool in the sand, it was insufficient for the supply of the party. Encamped at 6.0 p.m. The geological structure of this portion of the country is wholly dissimilar to any other part of North Australia we have yet traversed. Granite, porphyry, and slate are the prevailing rocks. The whole appear to have been subjected to considerable disturbance, as the slate is much broken and contorted, and in several parts altered by contact with the porphyry, and no definite strike or dip appeared to exist. The porphyry is of a red-brown colour, consisting of grey paste with crystal of felspar and angular fragments of slate and granite sometimes one foot in length. The granite contains little mica, and the quartz frequently is arranged in rhomboidal crystals nearly parallel to each other; it readily decomposes, and from the predominance of quartz forms a coarse gritty soil. Quartz-rock forms large beds and veins in the granite, and has a general trend north and south. It often contains crystals of mica, and therefore not likely to contain metals. In washing the sand of the river near Camp 83, only a small quantity of titaniferous iron remained after the removal of the quartz and mica. It was in this locality that the Gilbert Gold Field was afterwards discovered.

Latitude by e Pegasi 18 degrees 25 minutes 33 seconds.

30th September.

Moved the camp about one mile higher up the river to some small pools of water, and then with Mr. H. Gregory ascended the hills to the south of the camp. From the highest ridge the course of the river was visible for nearly twenty miles, trending first seven miles south-south-west and then south-south-east; at the bend a branch appeared to join from west-south-west, in which direction the country appeared very flat for fifteen or twenty miles, as only a few distant hills were visible; from north round to south-east the country was very broken and hilly, rising highest to the north-east, but the view was limited to eight or ten miles; south-east a valley opened through hills, and more distant ranges were indistinctly seen beyond. The whole aspect of the country was barren, rock forming the principal feature. Returning to the camp, collected a quantity of the clustered figs on the bank of the creek; this fruit is rather insipid.

1st October.

Steering an average south-south-east course from 7.40 a.m. till 2.40 p.m., camped on the right bank of the river, which first came from south-west and then from south-east, throwing off two branches to the south-west, and was thereby diminished to 100 yards wide at our camp; only one creek and some gullies joined from the east, although the country in that direction was hilly; the bed of the river was still dry and sandy; water very scarce. Slate, quartz, schist, granite, and trap are the principal rocks, and by their decomposition do not produce a soil favourable to vegetation, the country becoming more desolate as we advanced. The only trees which retain their verdure are those which grow on the banks of the river.