Adopting a palæontological classification, we may speak of the Arenig and Llandeilo beds as those containing the _Asaphus_ fauna, whilst the Caradoc and Ashgill beds possess the _Trinucleus_ fauna; this is the terminology employed by Angelin for the equivalent strata of Sweden. It must be noted that here the names applied are not those of absolutely characteristic genera, as was the case with those adopted for naming the Cambrian faunas, for both _Asaphus_ and _Trinucleus_ range through the beds of the system; but whereas _Asaphus_ is most abundant in the beds of the two lower series, _Trinucleus_ occurs most frequently in those of the two upper series. _Description of the strata._ The Ordovician rocks are found over large tracts in North and South Wales, in the counties on the Welsh border, in Lakeland and the outlying districts in the Southern Uplands of Scotland, and in detached areas in Ireland. There are three main types of deposit:--(i) the volcanic type, in which the ordinary sediments are associated with a large amount of contemporaneous volcanic matter, (ii) the black shale type, with a fauna consisting largely of graptolites, and (iii) the ordinary sedimentary type, in which we find alternations of grits, shales, and more or less impure limestones. We also find developments which are intermediate between any two or even all three of these types. The first type is characteristically developed in Caernarvonshire and Merionethshire, the second in the Dumfriesshire Uplands, and the third in the Girvan district of Ayrshire. The variation in the thickness of these three types of deposit is shown in the accompanying sections of the Caernarvon, Merioneth, Moffat and Girvan regions (see Fig. 17). [Illustration: Fig. 17. Showing the variations in the characters of the Ordovician deposits of the three principal types. Scale 1 in. = 1000 feet. A = Arenig. L = Llandeilo. C = Caradoc. The thickness of the Arenig rocks of the Scotch areas is unknown.] The North Welsh area gives two different developments of the Ordovician strata, one of which is much less volcanic than the other. In the Merioneth-Caernarvon area, two great masses of volcanic rock form the Aran and Arenig hills of Merioneth and the Snowdonian group of Caernarvon. The former are of Arenig, the latter of Caradoc age. The Merionethshire volcanic rocks consist of a great thickness of lavas and ashes of intermediate composition (anderites), associated with sandy and muddy sediments of no great vertical depth. The Llandeilo beds of this area are chiefly of the nature of black shales, while the Caradoc series is represented by volcanic lavas and ashes of acid composition (felsites) with a few thin interbedded sediments. A calcareous ash forming the summit of Snowdon is of importance as being on the same horizon as a limestone (the Bala limestone) found in the other North Welsh area. The Ashgill series is not represented in Snowdonia.