Later eruptive rocks, consisting of masses of syenite and granite, with still younger dykes of dolerite, andesite, diorite and felsite, have successively made their appearance, and add to the diversity of the igneous phenomena of this district. The question of the age of this isolated volcanic series is one of much interest, but of great perplexity. Though a resemblance may be admitted to exist between some of the slates and parts of the Cambrian system of North Wales, the difference between the Charnwood rocks and the undoubted Cambrian series of Warwickshire, only thirteen miles to the south-west, is such as to indicate that the former are probably older than the latter. While the Charnwood rocks have been intensely cleaved and crushed, those of Warwickshire have undergone no such change. The argillaceous strata in the one region have been converted into slates, in the other they remain mere shales. Though cleavage is sometimes irregularly developed, its rapid disappearance in so short a distance as the interval between Charnwood Forest and Nuneaton seems most explicable if we suppose that the rocks at the more easterly locality were cleaved before those towards the west were deposited. If this inference be well grounded the pre-Cambrian age of the Charnwood volcanoes would be established. But the argument is not conclusive. No fossils of any kind have yet been found in any of the old rocks of Charnwood.[81] Merely lithological resemblances or differences are all that can be used as a guide to the geological age of these masses. Mr. Watts has suggested that possibly the quartzite of Bradgate (No. 6 of the Charnwood groups) may be the equivalent of the quartzite which in Shropshire and Warwickshire forms the base of the sedimentary Cambrian formations. If that correlation could be established, the volcanic series below the quartzite in Charnwood might be regarded as representing the Uriconian volcanic series of Shropshire. [Footnote 81: Since this page was in type, Professor Lapworth has found a worm-burrow low down in the Brand Series, and one or two additional examples have since been obtained by Mr. J. Rhodes of the Geological Survey. These are the first undoubted organisms from the Charnwood Forest rocks. Mr. Watts, _Geol. Mag._ 1896, p. 487.] BOOK III THE CAMBRIAN VOLCANOES CHAPTER IX CHARACTERISTICS OF THE CAMBRIAN SYSTEM IN BRITAIN The Physical Geography of the Cambrian Period--The Pioneers of Palæozoic Geology in Britain--Work of the Geological Survey in Wales--Subdivisions of the Cambrian System in Britain.