The Torridonian system is composed of rocks which are largely of arenaceous character, the most prominent beds being formed of red sandstones, and the bulk of the fragments in them have clearly been derived by denudation from the crystalline schists, many of the beds being composed of arkose, where the quartz is mixed with a large proportion of felspar and often of ferro-magnesian minerals. The deposits are clearly sedimentary, and are as little altered as many strata of much more recent origin, only possessing structures produced by metamorphic action under exceptional circumstances. The detailed researches of the geological surveyors prove that the rocks of this system have a much greater thickness and are of more varied lithological characters than was previously supposed. The total thickness of the strata is over 10,000 feet, and the sandstones are associated with deposits of a muddy character, and with occasional bands of limestone; in these circumstances the discovery of fossils would excite no surprise, and in 1891 Sir A. Geikie announced the detection of "traces of annelids and some more obscure remains of other organisms in these strata," which have not yet been described[49]. These Torridonian strata furnish us with the most satisfactory group of Precambrian sediments yet detected in Britain[50]. [Footnote 49: An account of the subdivisions and lithological characters of the rocks of the Torridonian System will be found in the _Annual Report of the Geological Survey of the United Kingdom_ for 1893.] [Footnote 50: It has been recently maintained that some of the Torridonian rocks are of Æolian origin.] In the south-east Highlands is a great mass of crystalline schists of a less gneissose character than that of the north-west, to which Sir A. Geikie has applied the name Dalradian. Many of these schists will be found by examination of the geological map of Scotland to be separable into divisions, which by means of their lithological characters can be traced long distances across the country, and they present all the characters of sedimentary rocks, though they are associated with intrusive igneous rocks, and have undergone great metamorphic changes since their formation. Cambrian rocks have not yet been discovered immediately above them, though they are clearly older than Ordovician times, but the existence of rocks associated with them along their north-west borders, which in lithological characters closely resemble some of the rocks of the crystalline schists of the north-west Highlands, indicates the probability of their general Precambrian age. In some instances, the extreme types of metamorphism which they exhibit are the result of the kind of action usually termed pyrometamorphic as has been shewn by Mr G. Barrow[51]. [Footnote 51: Barrow, G. "On an Intrusion of Muscovite-biotite gneiss in the S.E. Highlands of Scotland, and its accompanying metamorphism." _Quart. Journ. Geol. Soc._, vol. XLIX. p. 330.]