For our present inquiry, however, the chief feature of interest in these discoveries is the recognition of a group of volcanic rocks underneath the quartzite. This group was named the "Caldecote Volcanic Rocks" by Professor Lapworth, who first recognized its nature and relations. Its rocks have been studied by Mr. T. H. Waller[128] and Mr. F. Rutley,[129] and have been traced upon a revised edition of the Geological Survey map by Mr. A. Strahan.[130] They consist of a thin series of well-stratified tuffs apparently derived from andesitic lavas. Their base is not seen owing to the fault which brings down the New Red Sandstone against them. They are surmounted by the quartzite, which at its base is conglomeratic and contains blocks of the tuff. A mass of quartz-felsite is possibly intrusive in these strata, and is associated with a diabase-porphyrite. In these rocks, but still more in the shales which overlie them, numerous sills of diorite and diabase occur. The total thickness of rocks from the lowest visible part of the Caldecote volcanic series to the base of the Coal-measures is probably between 2000 and 3000 feet. [Footnote 128: _Op. cit._ p. 323.] [Footnote 129: _Op. cit._ p. 557.] [Footnote 130: _Geol. Mag._ (1886), p. 540. In this paper full references will be found to the previous papers on the geology of the district. Jukes had recognized that the rocks below the coal-bearing strata were "older than the Upper Silurian, perhaps older than any Silurian," _Mem. Geol. Survey_, "South Staffordshire Coal-field" (1859), p. 134.] There can be no doubt as to the geological position of the dark fossiliferous shales and their underlying quartzite. The fact that the basement conglomerate of the quartzite is partly made up of the underlying volcanic series may possibly mark a wide difference of age between them, and may indicate that the eruption of the tuffs took place long before Upper Cambrian time. On the other hand, the tuffs have the same strike and angle of dip with the quartzite, and as Professor Lapworth admits, the break between them may not be of great moment. It is at least certain that the intrusive sills of the district are later than the tuffs, and later also than the sedimentary Cambrian groups. BOOK IV THE SILURIAN VOLCANOES CHAPTER XII CHARACTERS OF THE SILURIAN SYSTEM IN BRITAIN. THE ARENIG VOLCANOES The Land and Sea of Silurian time--Classification of the Silurian System--General Petrography of the Silurian Volcanic Rocks--I. The Eruptions of Arenig Age.