1869.] MURCHISON-SUTHERLAND GOLD-FIELD. 315
and limestones, which, thanks to the fossil discoveries of my friend
Mr. C. Peach, were proved to be of Lower-Silurian age. It was
also shown that these Lower Silurian rocks, in their extension eastwards
and dipping slightly to the east south-east, became more and
more crystalline as they trended in that direction, and that when
perforated by bosses of eruptive rocks, chiefly granite, they rolled
over in undulations in a highly metamorphosed condition, until they
were surmounted unconformably by the Old Red Sandstone and
younger deposits of the eastern coast.
In order to be able to reason upon the probable sources of the gold detritus which occurs in the gullies (valleys) of such hard crystalline rocks of Eastern Sutherland, it is necessary to recur to those transverse sections from W. to E. which I laid before this Society in 1858, together with a geological sketch map* of the Highlands, in both of which the true general order was first explained.
In referring to these, a chief point which is to be borne in mind is the peculiarity of the outline of the mountainous region of Sutherland and Ross, presenting a steep and rapid escarpment, down which the waters cascade to the western shore, whilst they flow along a gentle slope for many miles to the eastern shore. Thus the parting of the waters in this region lies within four or five miles, and sometimes less, from the western sea-shore, whilst it is from seventy to eighty miles from the eastern shore, in which long descent the waters flow over the metamorphosed Lower Silurian rocks, with occasional bosses of granitic and other intrusive rocks. Now, however transported (probably by floods carrying great masses of ice, caused by the melting of former glaciers — a view which, in a more extended sense, has recently been applied by Mr. John Campbell, of Islay†, who has examined the localities), it is evident that all the disintegrated materials of the great mass of the western and central Highland rocks must have been transported eastwards. I am, indeed, led to suggest that the gold debris found in the environs of Kil-Donnan and Helmsdale are the result of the abrasion of extensive masses of the granitic and metamorphic Lower Silurian rocks which, occupying wild interior tracts, extend eastwards to the district under consideration, where their broken materials have been lodged in the depressions of Eastern Sutherland. Reasoning in this way, and looking to the general character of the rocks between the west and east coast, I am led to think that certain valleys on the long eastern slope of Rossshire which accompany the line of Loch Shin and the river Oikel may also be found to be slightly auriferous. It is here also to be noted that the gold-bearing detritus of Sutherland occurs in a district which exhibits in its environs the most striking evidence of granitic eruption and of highly metamorphosed rocks.
Thus on the Helmsdale shore we have the grand mountain-mass of granite known as the Ord of Caithness, and on the north of
- See Quart. Journ. Geol. Soc. vol. xv., Section, p. 360, and Map, p1. xii.;
also vol. xvi. p. 215.
† See "Something from the Gold-diggings in Sutherland," by the author of 'Frost and Fire,' from 'Odds and Ends,' No. 22.
VOL. XXV.-PART I. Z
