argued by Professor Ramsay, Mr. Godwin-Austen and others), of lacustrine origin, — a conclusion supported by the paucity of organic remains in them, and the peculiar characters of the rock which forms their upper member.
§ 2. The Rhaetic ?
At the base of the Liassic series of strata in Sutherland, and immediately overlying the formation just described, there occurs a series of coarse sandstones with beds of conglomerate (fig. 14, c, page 143). As has been already pointed out, the pebbles in this conglomerate are not derived from the Old Red Sandstone or any other Palaeozoic rock, but in part at least, from the sandstones and cherty limestones of the Trias. The base of the Lias in Scotland, as in South Wales and many parts of France and Germany, is usually formed by similar conglomerates ; but on the Western Coast the Secondary rocks generally repose unconformably on the older Palaeozoic strata, such as the Silurian and Cambrian, and the conglomerates are made up of pebbles of those rocks.
These conglomerates and sandstones of Sutherland, which attain a considerable thickness, but are somewhat imperfectly exposed, have not exhibited any trace of those bone-beds sometimes found in equivalent strata ; nor, indeed, have they yielded any kind of fossil remains whatever. As indicating the slight oscillations of level, insufficient to produce unconformity, which marked the gradual transition from the Trias to the Lias, we may, with a strong show of probability, consider them to be of Rhaetic age. They are evidently a littoral deposit, and are overlain by, and graduate up into the series of estuarine strata constituting the base of the Lower Lias.
On the opposite or southern side of the Moray Firth there are a number of masses of strata, not in situ, but included in the Boulder- clay, which from their mineral characters have been variously referred by different authors to the Wealden, Purbeck, and Rhaetic. I have already pointed out how little weight should be attached to mere mineral characters in the determination of the age of the Secondary strata of Scotland ; and as the masses in question are completely isolated in the drift, we are reduced to the purely palaeontological evidence.
The most important of these masses is that of Linksfield, which was first brought under the notice of geologists by Dr. Gordon in 1832*. The peculiarities of the strata here were found by Dr. Malcolmson to be such as to lead him, in the year 1838†, to suggest that they were the equivalents of the Wealden or Purbeck — a view that was adopted by Mr. Duff, Mr. Robertson and other observers‡. On the other hand, Mr. C. Moore §, in the year 1859, pointed out the striking resemblances in the mineral characters and succession of
- Proc. Geol. Soc. vol. i. p. 394.
† Ibid. vol. ii. p. 667.
‡ Sketch of the Geology of Moray, 1842 ; Anderson's ' Guide to the Highlands,' 3rd edition (1851) ; &c.
§ Brit. Assoc. Rept. (1859) p. 264. Quart. Journ. Geol. Soc. vol. xvi (1860) p. 445.
VOL. XXIX. PART I. L
