NATURALISTS' FIELD CLUB

Evening Star, Issue 25893, 10 September 1946, Page 5

NATURALISTS' FIELD CLUB

GEOLOGY OF MOUNT HOLMES The following geological notes have been sent in by Mr H. Tily, in respect to a recent outing of the Naturalists’ Field Club to Mount Holmes:— From a geological viewpoint Mount Cargill is a flat-topped mountain like the other mountains around Dunedin. Its flat top extends from above the Leith Valley road to Mount Zion, but it is distinguished from the others in that three intrusions of igneous rock have formed lumps on this surface. These lumps are Mount Cargill, Buttar’s Peak, and Mount Holmes, and they have been formed by molten rock forced up from below into crevices in the adjacent rocks. It is. probable, that they did not reach the surface at that time, and being harder than the adjacent rock have since been exposed by denudation. Certainly Mount Holmes must have been formed underground, as its organ, pipe structure indicates a slow uniform cooling in which the heat passed in parallel lines to an extended plane surface. Such conditions conduce to an even and regular contraction of the mass, whereby it splits into cracks, in the direction of the passage of the heat, and thus forms long prisms in that direction. These prisms end in such shapes as triangles, quadrilaterals, and other shapes, but the pentagon and hexagon usually predominate. A perfectly even cooling would produce all hexagons, for much the same reason as the honey bee produces cells which are hexagonal in cross section.

Another and better example of such prismoidal structure in rocks is to be found on the' seaward side of Black Head. Such structure can also be traced in Buttar’s Peak and Mount Cargill, but it. .is there somewhat coarser and irrregular, and indicates more uneven cooling of the mass, perhaps because it was nearer the surface of the ground, or because the surface was broken or uneven.

A swamp surrounds Mount Holmes, due to the existence of ash beds in the adjacent rocks, which are impervious to water and prevent the water from soaking' downwards. It must, therefore, drain away sideways or remain until it evaporates.