GEOLOGY LIFE DURING ORDOVICIAN TIMES The abundant graptolite fauna of the Skiddaw Slates has been well worked out by Miss G. L. Elles, and her general conclusions have been already mentioned. It must not be supposed, however, that the whole of the fauna of these beds has been fully determined, as such is hardly likely to be the case for a long time to come owing to the great changes which have taken place in the character of the beds since they were deposited. Strong cleavages have been induced sufficient to convert the mudstones into slates, and the beds have also been invaded by intrusive rocks and much altered by contact-metamorphism. Before the close of the Skiddaw Slate period the volcanic eruptions which were to give rise to the overlying Borrowdale Volcanic Series had commenced, so that thick ash beds and lava flows alternated with the last phases of marine sedimentation. Many of the ash-beds have undergone a later cleavage development, and are at times almost indistinguishable from the true slates. The Coniston Limestone series has yielded a large number of fossils peculiar to the Bala Beds of North Wales. Amongst these are several corals, including Monticulipora {Favosites) fibrosa and Heliolites interstinctus. Brachiopods are especially distinctive, and include such well-known forms as Orthis calltgramma, O. porcata, 0. elegantula, Lepttena sericea, and L. {Strophomena) rhomboidalh. The Ashgill Shales are characterised by the trilobites, Trinucleus concentricus, Phacops mucronatus, and P. apiculatus, together with species of Orthis and Strophomena. A very complete list of fossils from various horizons is given in Dr. Marr's paper on the Coniston Limestone Series.^ It is needful to remember that the Ordovician strata of the English Lake District and North Lancashire are the equivalents of the vast mass of slates, grits, and limestones which in North Wales form the Arenig, Llandeilo, and Bala groups, and that it is also quite possible that the lower portion of the Skiddaw Slates may prove of Cambrian age and to belong to the Tremadoc or Lingula Flag series. SILURIAN Rocks of Silurian age form a broad fringe to the south of the Borrowdale Volcanic Series in the Lake District, the Ordovician beds already considered forming but a narrow ribbon between them. Almost the whole of North Lancashire north of a line drawn from Lindale and Ayside to Cartmel, Ulverston, and the Duddon is occupied by these rocks, and they stretch across the eastern half of Cumberland to Yorkshire. The series consists of shales or mudstones, flags and grits which reach a thickness of between 14,000 and 15,000 feet. They have been divided as follows: — Kirkby Moor Flags ...... 2,000 feet Bannisdale Flags . . . . . . 5,200 „ Upper C Coniston Grits and Flags .... 4,000 „ Coniston -J gtockdale Shales ■[ ^"""^g'^^ '^^^^ Group I, Graptolitic Mudstones . 200—450 Basement Bed The Basement Bed which at Austwick possesses the character of a calcareous conglomerate, rests unconformably upon the upper members of the Ordovician series,^ or, as near Souththwaite, upon a series of slates with gritty bands, which pass into rocks sometimes called ash-beds. Below the latter are flaggy slates passing down into the Coniston Limestone. At Skelgill and Pullbeck, near Ambleside, the place of the conglomerate is taken by grit bands and calcareous beds, whilst in other places it seems to be absent. A marked unconformity separates the basement beds from the under- lying Ordovician, and this is also accompanied by a marked Silurian fauna in the upper beds. STOCKDALE SHALES These consist of blue mudstones and calcareous and graptolitic shales, which are divided into — Browgill Beds and Graptolitic Mudstones. The Graptolitic Mudstones are of great interest notwithstanding the thinness of the beds, owing to the prevalence of graptoHtes. The dark shales or mudstones are especially prolific in species of graptolites, the chief zones being in descending order as follows : — Monograptus spinigerus Monograptus argenteus „ Clingani „ fimbriatus „ convolutus Dimorphograptus confertus 1 Geol. Mag., Dec. iii. (1892), ix. 108-1 10. ^ T. McK. Hughes, Geo/. Mag., iv. 352 (1867). 3
