GEOLOGY OF NELSON

Nelson Evening Mail, Volume LXI, 13 January 1928, Page 2

GEOLOGY OF NELSON

LECTURE BY DR. MARSHALL TEACHERS* SUMMER SCHOOL This morning's general lecture in the Teachers' Summer School programme was delivered bv Dr. P. M-arsliaJl, M.A., D.Sc, F.R.G.S., ex-Professor of Geology, Otago University. The lecturer was introduced bv the President (Mr F. Milner). The Boulder Bank was the, first feature of Nelson's geologv dealt with by Dr. Mai'shall. He had presumed that those students who visited Cable Bay yesterday noticed the long bank, of boulders* and also the cliffs of Peppin Island and Mackay's Bluff. There was no doubt that the millions of tons of gravel and rock contained in tho Bank had originated from Mackay's Bluff and Peppin Island. It had/ of course, taken some, considerable time for the material to be carried along ten miles to form ilie present length of the Bank. There had been at various times a great many theories attempting to explain the origin of the bank. It had been asserted that the material of the Bank was the terminal moraine of a huge glacier that had in rirevions ages filled the Nelson :valleys with a river of ice, but consideration of a few fads would show 'that this contention was untenable. If the rocks had come from the inland part of Nelgon then one would expect to find similar rock somewhere in the material, but rock of a like nature did not occur anywhere in the back country, while, it was the identical material of which Peppin Island and Mackay's Bluff were .made. The objection might ho raised that there was not sufficient evidence, to support this theory, hut it could be answered by reference to observations that had been made by Mr Davies and others now on the Cawthron Institute staff. Twenty years ago Mr Davies fixed the position of a long ridge of stones on the bank Bft. wide and 100 feet high. During the twenty years up to the present it had moved no less than 100 yards, which considered alongside some natural changes was the speed of an express train. . There had been, said Dr. Marshall, considerable difference of opinion with regard to the length of time that had passed over the face of the earth. The geologists used to contend that the earth wassoo million years old, while the physicists favoured a shorter period of 20 million years. The physicists argued that they knew what the approximate heat of the earth once was, and they knew what it was at the present time, and that as it was giving out heat**at a certain measurable rate they could calculate- its age by exact mathematics. The geologists answered this by saying that the sculpture of the earth's surface wrought by nature was also a clear indication of time. The controversy raged more or less heatedly until the momentous discovery of radioactivity. Radio-activity showed that there were substances in the interior of the earth continually giving out heat and keeping up the temperature of the earth. The physicists at once went to the other extreme and said that the earth was a 1000 million years old. The lecturer then' turned to a large diagrammatic panorama of Nelson and its surroundings that he had specially prepared. He outlined trie general situation of the city referring briefly to tho surrounding ranges and the Botanical hill with its stone marking the centre of Newx Zealand. A further feature discussed was the Port hills, which Dr. Marshall said were composed of gravels brought down by an ancient river that once had its source at the foot of the Hope Saddle and Lake 3 Rotoiti and Rotoroa. Beneath these gravels were sediments containing fossil shells of a form of life that had long since disappeared from the face of the earth. "A fossil," said the speaker, "is one of the most romantic, interesting arid stimulating to the imagination of all, the objects of nature, for from it one can build up a picture of a world long since vanished and peopled by all manner.of strange and monstrous forms of life." At .the period when these fossils under the Port hills were laid down the climate of the locality was much warmer and the land of New Zealand was restricted to a string of islands. ' i

There was one geological feature of Nelson which was more renowned throughout the world than anything else in New Zealand, and that was the Dun Mountain, which contained one of the classical features of the world's geology. The mountain was composed of dunnite, a material very exceptional in its chemical nature. This very rare rock was a silicate of magnesia without the potash and the phosphorus necessary for plant growth. This was the reason why the Dun Mountain was almost devoid of vegetation. How this particular kind of rock got to its present position was one of the mysteries that geological science had not yet fully probed. Dunnite was an igneous'rock of a very high density or ■ specific gravity of 3.2. It was three and a-fifth times as heavy as water. The. earth as a whole was five and a half times as heavy as water, but the material forming the earth's surface was only two and a half, times as heavy as water. As one proceeded deeper and deeper, then heavier and heavier materials were found and the Dun Mountain properly belonged at a depth of 200 miles below the surface. Another interesting rock occurring on the Dun Mountain and one which the speaker was fortunate enough to be > the first to discover was roadingite (as he had named it), composed almost entirely of white garnet. This rock was very interesting because of its occurrence in such a locality. Roadingite was very seldom produced from igneous action, that is from molten material. There was also to be found on the Mountain a lot of serpentine, a greenish stone formed by the action of Rainwater and the weather on dunnite. ;

In conclusion Dr. Marshall referred briefly to the general aspect of Nelson with' its wonderful system of valleys carved out bv the simple action of rainfall.