170 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Jan. 27,
There is abundant evidence that the latter rocks were very much
disturbed during the period in which they were formed; for we
found the gneiss in a most contorted state, and not unfrequently
containing extensive developments of quartz, both in veins and in
lenticular forms. None of these veins are ever found entering the
strata which succeed the gneiss and granite and rest unconformably
upon them. I take it that these strata are Silurian; Bain and all
his Cape contemporaries thought so. They are generally horizontal,
but not unfrequently are flexuous, and at times so faulty that there
maybe a difference of even 1000 feet in the elevation of strata which
at one time were continuous on the same plane.
The primitive rocks in the valleys where the gold has been found have been very much eroded by the rivers: in some parts it is no exaggeration to take it at 3000 feet depth; and it is common to find valleys of from 500 to 1000 feet in depth. It would appear to me a reasonable assumption that no part of the gold contained in the rocks that have been removed (chiefly in the form of mud and fine sand) has been carried away, but that it remains in the alluvions now traversed by the same rivers. Although in the parent rock the quantity may be imperceptibly small, it may be found in more abundant traces in the debris, which throughout an extensive valley will show the course the river followed in the remotest periods of its existence. The features peculiar to one river will apply to all which traverse the primitive rocks; the same action has been alike effectual in all. In the faulty state of the Silurian strata to which I have alluded, there is ample proof that the primitive rocks on which they unconformably rest have suffered great displacement, and the relative levels of many localities have been greatly altered. This, no doubt, would have resulted, in the first instance, in forming valleys of elevation, upon which the denuding agency, down to the present time, has been operating. The point I wish to bring out is, that the gold contained in the primitive rocks will be found collected in the alluvions, and that hence, the amount being very small, it may be imperceptible in the rocks in situ, although quite perceptible in the debris along the course of the river.
I was not able to ascertain from Mauch, when he was here some months ago, whether the occurrence of the gold in the interior has the same relations as in this colony, or whether it bears comparison with that of the Ural, or Australia and America, where it would appear to pervade the ramifications of veinstones of quartz into the strata.
Discussion.
Mr. David Forbes was glad to find that Dr. Sutherland corroborated his views as to the occurrence of gold in two ways:—
1. In auriferous granite, as in Wicklow and elsewhere.
2. In eruptive diorite, a basic rock without free quartz, and certainly of postoolitic date, almost always accompanied by copper-veins. Most Californian gold-veins are connected with this class of rocks.
