Hunter Island, Sullivans Cove, Hobart, was used as jetty. [Painting by G P Harris, 1804. Mitchell Library, Sydney]
Cataract Gorge, Launceston
Photographs by David Leaman
Darwin's
observations
Little more is heard of the rocks of Van Diemen's Land until Charles Darwin arrived on HMS Beagle in February 1836. He described "extensive basaltic platforms" around Storm Bay and "huge angular masses of naked greenstone" at the summit of Mt Wellington. He noted that the greenstone produced a magnetic effect.
Darwin also commented that the greenstone appeared to be faulted against horizontal strata near Port Arthur but these are distant sightings. His use of descriptive terms such as syenitic greenstone on the Queens Domain in Hobart were as good as he could achieve since the dolerite there is coarse and weathered.
Dolerite is an intrusive igneous rock; that is, one which has "invaded older rock and may displace or dilate the pre-existing rock", contrasting with "extrusive", where magma reaches the Earth's surface and erupts from a volcano as lava flows which then cool. It took many years to agree on this position.
Darwin was able to separate dolerite (greenstone is his word) from basalt or lava on the basis of texture, mineralogy or field evidence such as gas vesicles and crystal size at Sandy Bay. In other places, such as Cornelian Bay, he simply noted that the greenstone was syentic or granitic.
Darwin provided the first hint that his greenstone might not be volcanic when he described a narrow dyke in Lenah Valley where the introduced sandstone had been altered by heat and and the layers or beds appear to have been tilted. The problem for Darwin was whether his greenstone dyke was really greenstone and connected to larger areas of greenstone nearby or whether it was a feeder vent for a volcano now eroded away.
He walked across two igneous contacts in the Bellerive-Howrah area and was confused. He did use the words "altered" and "metamorphised" when referring to the Permian siltstones near Knopwood Hill and linked the changes to the greenstone.
He was also able to write: "… lower strata of the second series generally rest on a coarse greenstone … which has burst through them in a fluid state …" He had recognised that the rock was intrusive of the Permian rocks.
What a pity this comment was never published. We would have been spared a lot of argument and some inanity in the decades that followed. ¶
Note: For more of that inanity and subsequent confusion regarding dolerite and how Tasmania's abundance of this unusual rock influenced our history since settlement, read David Leaman's book The rock that shaped Tasmania, from which the above extract comes. Read our review of this fascinating book here.
::: BACK IN TIME
A rock and
a hard place
PART TWO | The second voyage of James Cook and his second in command, Tobias Furneaux, to the Pacific began well but soon became disorganised due to storms and boat damage. Furneaux put in to Adventure Bay for water and wood. He applied the named Fluted Cape to the great cliffs at the southern end of Adventure Bay.
James Cook visited the bay and anchored in the shadow of the great cliffs in January 1777 on his third and final voyage to the Pacific but he, like William Bligh who was with him and who returned on the Bounty in 1792, was more interested in wood and water than rocks.
Recherche and Esperance under command of Admiral Bruni D'Entrecasteaux arrived on April 21, 1792 with initial landfalls near South East Cape and anchored on the sandy bottom of Recherche Bay. They sailed east on May 28 past more great cliffs to find the long lost La Perouse. Cape Raoul was named for the pilot.
D'Entrecasteax returned some months later to re-provision and between January 21 and February 27 1793 his expedition named Cape Frederic Henry and Cape Cannele; the first is named Cape Queen Elizabeth today and the second had already been named Fluted Cape by Furneaux.
The names represent a conjunction of meanings in two languages, meaning a cape which is fluted or piped. Both names remain; Fluted Cape is applied to the northern end of the cliff and Cape Connella to the southern end.
He profiled a high mountain to the west, named it Skiddaw (now Mt Wellington) and commented on the "coxcomb", referring to the Organ Pipes.
The botanist La Billardiere and the chaplain Ventenat made a few comments on the geology and there are many references to a hard, grey rock or, in other places, to a rock with reddish skin when weathered. The term granit (granite), meaning a coarse, crystalline rock in their vocabulary, was also applied to the coarser versions of the hard grey rock.
One month after D'Entrecasteaux had sailed for home, Lt John Hayes cruised into Storm Bay aboard Duke of Clarence and Duchess. He felt the country north of Risdon was like the Lake District of England and named the river, Derwent. He profiled a high mountain to the west, named it Skiddaw (now Mt Wellington) and commented on the "coxcomb", referring to the Organ Pipes. The term Cockscomb is attached to a columnar ridge near Mt La Perouse today.
Bass and Flinders completed a circumnavigation of Van Diemen's Land in Norfolk in 1798 and landed beneath a great flat-topped mountain (Mt Table now Wellington). Bass set off to climb it on Christmas Day. Flinders gave the name Cape Basaltes to Cape Raoul because of the appearance of the columns but was perturbed at their size; far larger than in any European lava he had seen.
Cape Raoul
Citizen Baudin's expedition arrived in 1802 and the scribe for the voyage, Peron, described a basaltic rampart (Organ Pipes), implying a volcanic origin for the Montagne du Plateau (yet another name for what is now Mt Wellington).
The columns at Cape Hauy, the far eastern projection of Tasman Peninsula, were compared to a rank of organ pipes and the feature was itself named for the respected French scientist and mineralogist, Hauy.
When Col David Collins arrived in February 1804 and moved the new Hobart settlement from Risdon to Sullivans Cove, there, in the middle of the bay, stood a small tied island of dolerite. He landed his men and stores on it and all then walked the sandy spit to the shore. This is Hunter Island, still to be seen beneath the docks at the eastern end of the bridge across Victoria Dock, though the top was hewn level for wharfage and storage and the material used as fill.
No one could escape this rock — it seemed to be nearly everywhere.
Over the next three years, as attempts were made to found a viable settlement in the north of Van Diemen's Land, dolerite and its products determined what happened. The York Town settlement was rather arid and established in covering materials largely derived from dolerite while George Town site was founded on dry dolerite hills with limited soil.
The more fertile plains around Launceston were supplied by the rivers which had eroded dolerite and produced gorges. The dolerite rocks of the Cataract Gorge impressed all. No one could escape this rock — it seemed to be nearly everywhere.
Botanist and naturalist Robert Brown, who had sailed with Flinders in Investigator, visited in 1804. Brown explored around Mt Wellington and climbed to the pinnacle several times where he noted that "the perpendicular rocks atop Table Mountain affected the compass needle". This is the first recorded geophysical observation related to the dolerite although it is likely that de Rossel, a fine scientist travelling with D'Entrecasteaux who made observations of the magnetic field, must have noted similar responses at Recherche Bay. ¶
Mars Bluff, north Bruny Island