GREAT DYKE

Wairarapa Times-Age, 24 June 1940, Page 3

GREAT DYKE

NATURAL FEATURE IN SOUTHERN RHODESIA. GEOPHYSICAL RESEARCH ARRANGED. Arrangements have been concluded between the Government of Southern Rhodesia and Mr. Oscar Weiss, consulting geophysicist of Johannesburg, for the scientific investigation of the Great Dyke in Southern Rhodesia. The problem of this dyke was discussed by the retiring president of the Geological Society of South Africa, Major B. Lightfoot, recently in his valedictory address and the geophysical investigation is the direct result of his suggestion for more research.

The Great Dyke, a colossal intrusion of basic and ultra-basic rocks, with a width of two to eight miles, runs straight for a distance of over 300 miles across the north-eastern part of Rhodesia. According to Dr. Alex, du Toil its great width is against the body being a true dyke with vertical walls descending vertically into the depths; its enormous length is scarcely consistent with the definition of a laccolith, while its even breadth and symmetrical habit do not favour a sill-like nature.

Apart from the scientific interest in the work it may be pointed out that the seven layers of black chromitite of the Great Dyke are of economic importance. Tn these investigations the Swedish gravimeter of Professor Ising will be used by Mr. Weiss. This instrument reacts on the density dilTercnces between rocks. These differences are especially high between the heavy basic rocks of the Great Dyke and the lighter surrounding granites. The gravimeter cannot give the same amount of detail as the torsion balance which can detect the hidden structure of contacts between rocks of different densities, but the gravimeter can be used on hilly areas. Under those conditions the torsion balance would be handicapped.

These instruments were used in the geophysical investigation of the Free State gold areas.