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Argillite, syn. Common Argillaceous Slate (Kirwan); Clay-slate (Jameson); Shiste ArgilLeux (Bronchani).
Colour a greyish black, with a bright silky lustre; substance opaque, with a smooth surface; adheres to the tongue, and yields a strong argillaceous odour: the streak is greyish; the structure foliated, the folia? separating easily. The fragments are tabular, thin, shining, and friable.
Localities. — In New South Wales it is found about the Upper Hunter, the Vale of Clywd, the west of Mount King George, Badger's Brush, the western side of the dividing range, the north side of Argyleshire, Lake George, Gidley East, Long Swamp, Bango Range, at the Murrumbidgee River, Mount Kosciuszko, Pinnabar, Lake Omeo, Thompson's River, and in Gipps Land, on its N.W. and S.W. extremities.
In Van Diemen's Land, between Piper's River and George-town, where it is associated with mica slate and siliceous slate. Its strata here are nearly perpendicular; it extends uninterruptedly for about four miles in a southerly direction; crops out again about Miller's Bluff, associated with the same rocks, and finally disappears with them. The second locality at which it is found is Emu Bay East, where it is seen alternating with siliceous slate. Its strata here also are nearly vertical, and strike in the same direction as those of the former locality: it sinks however under the basalt of Hampshire Hills; reappearing at the foot of Mount Arrowsmith, (between Lake St. Clair and Frenchman's Cap,) where it is associated with mica slate, both lying in a vertical position. Has the appearance of roofing slate, and has often been mistaken for it in Van Diemen's Land; but the presence of mica, and its avidity for water, render it unfit for roofing.
General Remarks upon the First Epoch.
An examination of the evidences which prove the eruption of the crystalline rocks, shows that there were various degrees of intensity with which the expansive forces acted during that eruption. From the unequal heights which, under these circumstances, the crystalline rocks naturally assumed, results a want of uniformity in the inclination of the uplifted stratified crust, and a difficulty in assigning any prevalent dip to the sedimentary rocks.
Thus, at the highest point of elevation, Mount Kosciuszko (6500 feet), mica slate, and siliceous and argillaceous slates are vertical, and attain the height of 3200 feet; which is the case also on the western side of the dividing range, between Lake St. Clair and Frenchman's Cap (Van Diemen's Land). About the Trafalgar River, where the granitic floor has an elevation of only 1400 feet, siliceous slate has a dip of 45°. At Manes Range, between the rivers Murray and Murrumbidgee, the upheaved strata are nearly horizontal.
The best sections of the stratified masses (on the dividing range, Van Diemen's Land), between Lake St. Clair and Frenchman's Cap, also on Eldon Range and Ben Lomond, tend to prove that, of the stratified rocks, mica slate, being the nearest to the crystalline rocks, and following all the contortions of the base, forms the oldest portion, of the crust, and that siliceous and argillaceous slates, which rest upon it, are the next in order of superposition.
Which, among the crystalline masses, claims a priority over the rest in point of age, cannot be ascertained with any certainty. The geological evidences that exist in New South Wales and Van Diemen's Land, (Vale of Clwyd, Rathurst, Mount Kosciuszko, Lake Oineo, Ben Lomond, and the Western Range,) seem to prove that the incandescent granitic matter was the first to appear after the breach of the submarine crust; that it was on the granitic talus, that quartz rock and sienite forced their way to the surface; and that, upon the latter rocks, serpentine, porphyry, and greenstone made their appearance. Thus, about Bathurst, quartz rock overlaps granite; and on the Honeysuckle Range, porphyry overlaps sienite; on Mount Kosciuszko, S. W., granite is seen forming a base 2000 feet above the level of the sea, upon which sienite and quartz rock attain a further elevation of 4500 feet. Again, in Van Diemen's Land, sienite rests upon granite, and greenstone upon sienite. In the dividing range, between Lake St. Clair and Frenchman's Cap, the base is granitic up to the elevation of 1800 feet, upon which base quartz rock, massive such as has been described, rises in towering masses to 3200 feet more.
The fact of an alteration of the stratified rocks in contact with the crystalline masses, having in some instances taken place, is obvious in the region here described, though it cannot be as yet satisfactorily traced to its proper cause. Mica slate at times has its two ingredients, quartz and mica, perfectly and widely separated, exhibiting an irregular, nodular aggregation; at other times their intermixture is perfect and laminated: in both cases mica slate appears in contact with granite. The arenaceous sedimentary rock hkewise presents a fused and homogeneous mass of granular quartz rock, in which the naked eye distinguishes the grains, although the interstices are obliterated. Such rock is also sometimes found in contact with porphyries, though at other times it is widely separated from any kind of igneous rock.
The extent which the stratified rocks occupy in
the two colonies is trifling. They are confined to a small zone, and have been much abraded, to furnish materials for the formation of subsequent rocks.
The crystalline masses may be said to form almost all that tract of the two colonies which is coloured pink in the annexed map, and may, throughout, be described as rising to higher level than the sedimentary rocks. Amongst the former, granite, sienite, and quartz rock preponderate. The first-named rock constitutes nearly the entire floor of the western portion of New South Wales, to the entire exclusion of mica slate and gneiss, and extends far into the interior of New Holland, in masses of mammillary, tuberous, globular, or botryoidal form.
In many instances, these masses possess a character analogous to that which the same rock assumes in Central Asia, and sometimes, as is the case in the tract of country lying N. E. of Wellington Valley, between the estate of Guantewang and that of Mr. Montefiore, they present so striking a resemblance to the granitic masses found in the environs of Altai, that the graphic description of that locality furnished by Humboldt may serve to delineate also the abovementioned region of New South Wales.
"Nulle part, dans l'un et dans l'autre hemisphere, je n'ai vu des granites qui oflrent plus le caractere des roches d'6ruption ou d'epanchement que les granites qui entourent le massif de 1'Altai. Ces roches, isoldes, comme le seraient des porphyres ou des basaltes, sont depourvu de gneiss et de mica shiste. — Elles s'elevent dans la steppe au pied des raontagnes Alpines, sous les formes le plus bizarres. Lorsque de la steppe de Platovsk, oh on commence a distinguer a l'orizon les neiges des Alpes Tigirezk, on monte vers les bords rocheux du Lac de Kolyvan, on est frappe de ces eruptions de granite, qui, sur plusieurs lieux carries sortent d'un sol entierement uni. Les rochers sont tantôt alignés, tantôt dispersés dans la plaine, affectant les formes les plus bizarres de mures étroits, des tourelles ou des polygones. — Les buttes les plus petites ressemblent à des tribunes, à des sièges, ou à des monuments funéraires. — Ce qui donne surtout une phisiognomie étrange à cette contrée, c'est le contraste de hauteur et de volume des masses granitiques. — Les unes ont quatre à cinq cent pieds d'élévation, comme la Vysokàîa Gora; les autres atteignent à peine sept ou huit pieds d'élévation, et rappellent les petites soulèvements volcaniques qui hérissent ces plaines, que dans l'Amérique espagnole on désigne sous le nom de Mal-pays. Arrivé au village de Sauchkina ou Sauchka, nous nous
trouvâmes comme au centre de ces éruptions.
"D'autres formes plus extraordinaires encore se présentent dans les rochers granitiques qui se sont soulevés le long de la pente méridionale de l'Altaï, entre Boukhtarminsk, Narym et le poste Chinois de Baty. Ce sont ou des cloches et des hémisphères aplatis, ou des cônes placés au miheu de la plaine du Haut-Irtyche, cônes terminés le plus souvent par des épanchements latéraux en forme de murs très bas et très allongés. On dirait d'une coulée, effet de la fluidité de la matière sortie sur une crevasse."
Asie Centrale, vol. i. p. 297.
The rocks of this epoch, represented by the yellow colouring in the annexed map, are characterized by a group of different crystalline and sedimentary compounds, resting incumbent upon those just described, and which in Terra Australis contain the first record of organic life.