commensurate with the force and volume of these floods, and must tend materially to raise the alluvial flats in the low country and, eventually, to fill in the Gippsland lakes themselves. A deposit of an inch or more of silt and slime on the river-flats after one of these floods is not unfrequent; and I have seen instances where it amounted, in places, to feet.
The progress of settlement, by the formation of tracks and roads, the clearing of land, and even the paths made by cattle and horses, tends to drain the land more rapidly, to concentrate the flood-waters, and to increase the amount of sediment carried down from the higher grounds.
Within the last few years and, indeed, contemporaneously with the wet seasons I have mentioned, the wide flat gullies commonly seen in parts of the mountains, and especially near Omeo, have, almost without exception, been cut back into deep channels by the rain. The black soil has been cut into so as to form narrow deep ravines, locally known as "washouts," and which extend occasionally miles back into the hills[1].
Classification of the Subject.
I have endeavoured to classify the various formations in the subjoined tabular form in accordance with my present knowledge. The first list exhibits them stratigraphically; in the second list I have shown the Igneous and Metamorphic rocks as I conceive their geological age to be.
Fig. 1.—Diagrammatic Section across the Australian Alps in North Gippsland.
|
a. Silurian. b. Metamorphic crystalline schist. c. Granite. d. Quartz-porphyries. |
e. Middle Devonian. f. Upper Devonian. g. Carboniferous. h. Mesozoic Coal-measures. |
i. Marine Tertiaries of Gippsland. k. Tertiaries of the Murray River. l. Volcanic. |
- ↑ There are at present very few data available relating to the annual volume of water poured into the Gippsland lakes, or the amount of sedimentary matter deposited in them. Some idea may, however, be formed from the following statement, which is extracted from the "Report on the Physical Character and Resources of Gippsland," by the Surveyor-General and the Secretary for Mines (Melbourne, 1874, p. 29):—" The quantity of water which the rivers, their feeders, would pour into the lakes during ordinary weather, in the months of November, December, January, and February, is, according to estimates formed by us from data obtained by the late Mr. Dawson, 16,132,500,000 cubic feet, representing a depth of four feet and half an inch over the superficial area of the lakes."
