Their highest beds contain Dictyonema sociale, a fossil which passes up into the succeeding Tremadoc Slates.
The rest of the local succession of the Cambrian rocks is hidden by the unconformable overlap of Silurian.
The Hollybush Sandstone and the overlying sandy shales contain numerous intercalated volcanic rocks, some of which are of the age of the surrounding strata, while others are intrusive.
Nuneaton District.—In the neighbourhood of the town of Nuneaton in eastern Warwickshire, a strip of Cambrian rocks, about eight miles in length by one in breadth, has been detected within the last few years. The rocks consist of volcanic ashes, quartzites and thin-bedded shales, pierced by dioritic dykes. These strata were formerly mapped as altered Millstone Grit and Carboniferous shale, and their Cambrian age has only been recently demonstrated by the discoveries of Birmingham geologists. The complete succession is as follows:—
(1.) Caldecote Volcanic Group.—Well-bedded tuffs and volcanic ashes (see ante) with masses of Quartz-felsites and diabase.
(2.) Hartshill Quartzite.—Thick-bedded quartzites, with occasional layers of sandy shale.
(3.) Stockingford Shales.—
(a.) Lower Division.—(Obolella Beds) Purple and green shales with Obolella Salteri, Lingulella pygmea, Lingulella lepis, Acrotreta, Protospongia, &c.
(b.) Upper Division.—(Agnosias Beds) Grey and black shales with Agnostus pisiformis. Beyrichia Angelini and Lingulella Nicholsoni, in the lower zones, and Spheropthalmus alatus and Dictyonema in the upper zones.