{ Trilobites { Black and dark shaly, grey } { (Olenus, { and brown slaty flagstones } { Conocoryphe, Lingula { and sandstones, with siliceous } 6,000 { Paradoxides, Flags { grits and quartzites } { Fig. 17); { } { Brachiopods; { } { Cystideans. CAMBRIAN GROUP. { Llanberis slates, with sandy } 3,000 Annelides. Cambrian { strata } { Harlech grits 6,000 Oldhamia. LAURENTIAN GROUP. Upper { Stratified, highly-crystalline, } 12,060 Eozoon. Laurentian { and felspathic rocks } Lower { Gneiss, quartzite, hornblende } 18,000 None. Laurentian { and mica-schists } UPPER SILURIAN PERIOD. UPPER SILURIAN GROUP. Lithological Characters. Thickness. Fossils. { Passage Beds, Tile-stones, and } { Sea-weeds, { Downton sandstones, at the } 80 { Lingulæ, { base of the bone-bed } { Mollusca. { { { Micaceous, yellowish and } 700 { Crustacea and Ludlow { grey, sandy mudstone } { Fish-remains. Rocks { { { Argillaceous (Aymestry) limestone } 50 { Crinoids. { { { Argillaceous Shale with impure } 1000 { Mollusca of { limestones } { many genera. { Argillaceous or semi-crystalline } { Mollusca of { limestone } { many genera. { } { { Argillaceous shales, in places } { Echinodermata; Wenlock { slaty } 3000 { Actinozoa; Rocks { } { Trilobites. { Woolhope Limestone and } { { occasional bands of argillaceous } { Graptolites. { nodules } { Upper { Grey and yellowish sandstones } { Pentamerus Llandovery { (occasionally conglomerates) } 800 { oblongus, Rocks { with bands of } { Rhynchonella, { limestone } { Orthides, &c.