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Geology

I’d like to comment on the letter "Geology" (Jardine, Osborne, Volk) in PAN 28.6.

While the 1:50 000 geology sheets and descriptive booklet are a handy guide, the lay person nevertheless has difficulty in identifying different rock "species" in the field. The difference between sedimentary (uniform, non-crystalline matrix) and igneous (crystalline matrix) is often missed when dealing with the finer-grained basalts and dolerites. Certainly, in the Cape, the sandstones stand on their own - they are coarse textured and look like sand grains which have been compressed together. Limestone, on the other hand, again in the Cape, looks different from sandstone (although both are sedimentary) and often lacks the sand-grain structure of the latter, and in many cases contains coarse shell fragments

To assist the trio with their query, I suggest they establish the general geology of the area from available maps, and use mild acid (0.5 M hydrochloric is ideal) to test for free lime - the rock will "fizz" as it produces carbon dioxide. For the chemists amongst you:

CaCO₃ + 2HCl è CO₂ + H₂O+ CaCl₂

CaCO₃ is the major compound in limestone, dominating the composition of marine shells.

There most certainly is a need for a handy publication on the geology and soils of the Cape region - perhaps the Geological Society could be persuaded, at least to offer a seminar on the field identification of common rock-types in the area.

A Barrie Low, Kirstenbosch

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