THE COALS OF AUSTRALIA.
The coal formations of Aus tralia are as peculiar as most of its productions. It will be no ticed that the coal-seams are beneath the conglomerate, instead cf above it, and that the heavy sandstones corresponding with the " old red" are the superior instead of the inferior strata. The fossiliferous sandstone, upon which the coal formations rest, appears to be of the Devonian system; but there is still a great liversity of opinion in regard to the age of the Australian coal. several eminent geologists place Lt among the subcarboniferous cocks, or false measures, beneath cur true coal formation. Others glace it in the Permian forma :ion, or above the true coal mea iures; while many practical men are inclined to place it among the productions of the Carbonif ?Tons era. But it is evident, :i.om the thin and rather uncer tain character of the seams, and their position below the conglomerates and yeavy sandstones, that the coal of the New South Wales formation belongs ;o the lower coal series of the English, or our proto-carboniferous era.
The Carboniferous limestone has been discovered some distance in the interior, but its position in relation to the coal had not been determined. There is some probability that it is synonymous with the conglomerate, which exists above the coal, since this rock is made up of fragments, and is much the same as our conglomerate where it commences its metamor phism into lime.
Immediately over the "Sidney Sandstone"—which is from 1000 to 1400 feet thick—an immense deposit of slates and shales, intercalated with thin coal-bands, is found in all the basins or depressions of the great sandstone. This may be the true coal formation; but Nature, having exhausted her stores of carbon at an earlier period, produces but barren measures now.
The existence of coal in the upper measures is doubtful though the developments are but limited. What may yet be found in the interior of this vast continent—island we can scarcely call an area of 3,120,000 square miles—it is impossible to say.
But the fact that this coal formation—always beneath the Sidney sand stone*—is found extensively over a great portion of Australia, leads us to conclude it to be the chief coal formation of that country. If the coal existed above the sandstone in any valuable quantity, it would have been discovered at some of the many localities where the lower formation is developed.
The coal area of New South Wales, or that portion of it near Sidney, on the Hunter River, and Woolongong, on the Nepean River, is computed at 15,000 to 16,000 square miles. But this coal, accompanied by the great Sidney sandstone, is also found at Victoria, in Western Australia, Kergue len's Land, New Zealand, and Van Diemen's Land, or Tasmania. It has
been found at many points over this wide range of territory,—in fact, so general and extensive that the coal area of Australia may yet vie with the wide fields of the United States. Of its great extent there can be no doubt; but of its comparative commercial value there is less certainty.
It will be noticed by the analysis and measures given further on, that this coal is by no means valueless or unavailable. The seams are much mixed with slate and dividing bands, but the coal is generally good and serviceable, and of immense value to the steam navigation and commerce of that remote quarter of the globe.
A vast mountain-chain bounds the eastern coast of Australia, some 20 to 30 miles inland, but is prominent from Tasmania to North Australia, in a general north-and-south direction. It is known as the Blue Mountains.
Its crest or axis is of granite, and is flanked by gneiss, or metamorphic rocks, pierced by syenite, greenstone, basalt, trap, At some low points the superincumbent Sidney sandstone overlaps both the gneiss and granite rocks, apparently in the ancient basins, and now forms vast plateaus or basins of coal formation ; but generally the coal lies on the sea-face or Pacific slopes of the mountain-range. To the west of this range, behind Sidney, are the famous gold-regions of Aus tralia ; and there we would not expect to find coal. We presume, there fore, the coals of the east are chiefly confined to this slope. In Western and Middle Australia both the physical and lithological conformations may be different.
In the vicinity of Sidney the strike is northeast and southwest, and the coal generally exists in long trough-like synclinals, bounded by sharp parallel anticlinals ; but the dip is by no means uniform : it ranges from one to thirty degrees, and leans to every point in the compass.
Between Newcastle and Woolongong (on the Hunter and Nepean Rivers, south of Sidney) the basin is nearly 150 miles wide, in a right angle or transverse direction to the strike. Its extreme depth is calculated at 5000 feet, and its average dip one degree ; but the undulations are such that abrupt dips are frequently met with, and numerous trap dikes, or veins of porphyries, greenstone, and basalt, burst through the formation. Those frequent volcanic interruptions are "troubles" to the miners, and greatly impair the value and productiveness of the coal-field. In the vicinity of those dikes the coal is considerably altered, and is frequently changed to a coke.