Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/263

1869. SUTHERLAND—AFRICAN AURIFEROUS ROCKS. 169 4. Note on the Auriferous Rocks of South-eastern Africa. By Dr. Sutherland, Surveyor-General of Natal.

(Communicated by Sir R. I. Murchison, Bart., K.C.B.)

In communicating the chief contents of a letter which was addressed to me by Dr. Sutherland, I beg to point out to the Society that the author seems to have arrived at some of the same conclusions as those deduced by Mr. David Forbes from his examination of the gold-bearing rocks of South America, which he described as resulting from the action of two very distinct classes of igneous rocks. (See Geol. Magazine, vol. iii. pp. 385 &c., Sept. 1866).

Rod. I. Murchison.

Natal, South Africa, Sept. 8th, 1868.

A young man of the name of Parsons, who made a journey with me in March last along the southern rivers of this colony, washed some iron-sand, which gave distinct traces of gold. Since his discovery several persons have also succeeded in finding traces by the same means; but in no case has it been found in remunerative quantities; the particles never exceed the tenth or twentieth part of a grain in weight; the form is rounded and pitted, very much the same as the Australian "pepitas," or nuggets, but visible only with a magnifying power.

Parsons has been temporarily employed by the Government to explore the rivers, with a view to settle the question whether or not the metal exists in such quantities as will pay labouring men fair wages while employed in its extraction.

I have gone carefully over chap. xix. of your fourth edition of 'Siluria;' and the conclusion to which I am driven is this,—that the noble metal is in such a state of diffusion among the gneiss and granite (for these are the rocks that yield it) as to be imperceptible to the eye, and not in any way occurring in ramifications and small veins. You will recognize Humboldt's words as you give them at page 474.

In no part of the South-African promontory that I know do the hypogene rocks protrude into the Secondary formations. It is true we have a most abundant development of erupted rock, chiefly basaltic greenstone, which has pierced all the strata except the small patches of "chalk." Bain's Dicynodon-strata, which contain the coal, are abundantly impregnated with it. In one locality, the Insizwa Mountains in the basin of the St. John's River, its mineral character seems to resemble diorite; and there, along the line of contact with the Secondary strata, it contains various ores of copper, which have been found to contain about 100 grains of gold to the ton of ore. But this basaltic rock is much more recent than the basaltic rock we found in the primitive rocks; the latter appear to have settled down into perfect repose when the Palaeozoic strata were undergoing a succession of invasions from eruptions of igneous matter, which in no instance assumes the type of the granitic or gneissose rocks.