Although the chief surviving records of Cambrian volcanic action in Britain are found in Wales, there is no evidence that the volcanoes of the period lay chiefly in that region. It is certainly a suggestive fact that, in the few districts where Cambrian strata appear from under younger formations in England, they are generally accompanied with igneous rocks, though the age of the latter may be older or later than the Cambrian period. If the oldest Palæozoic rocks could be uncovered over the English counties, a more abundant development of volcanic materials might be laid bare than is now to be seen in Wales. Taking, however, the extremely limited exposures of Cambrian strata, we find two tracts that specially deserve attention. Reference has already been made to the ancient eruptive rocks of the Malvern Hills, the antiquity of which is proved by the position of the Cambrian fossiliferous strata that overlie them. But these strata themselves include certain igneous rocks which point to a recrudescence of eruptive energy in a far later geological period. Nearly half a century has passed away since John Phillips mentioned the intercalation of igneous rocks in the series of strata which is now classed as Upper Cambrian in the Malvern Hills. Since that date hardly anything has been added to the information which he collected. The existence of a group of rocks of such high antiquity, asserted to be of truly volcanic origin, and the precise horizon of which could be fixed by the stratigraphical aid of organic remains, seems to have almost dropped out of sight. Phillips noted the occurrence of what he regarded as truly volcanic materials in the Hollybush Sandstone and the overlying dark (Lingula) shales, and he clearly recognized that a wide difference of age separated them from the far more ancient igneous rocks of the central core of the chain. The Hollybush Sandstones were observed by him to have "often a trappean aspect and to be traversed with felspathic dykes." He found the overlying black shales to include "layers of trappean ashy sandstone." But it was at the top of these shales that he obtained what he regarded as the most conspicuous evidence of contemporaneous volcanic action. He there encountered a zone of "interposed trap rocks" varying up to 50 feet in thickness, consisting of "porphyritic and greenstone masses, which, erupted from below, have flowed in limited streams over the surface of the black shales." He recognized amygdaloidal and prismatic structures among them.[123] The position of these eruptive rocks is shown in Fig. 45. [Footnote 123: _Mem. Geol. Survey_, vol. ii. part i. pp. 52, 55; also Horizontal Sections of the Geol. Survey, Sheet 13, No. 8, and Sheet 15. Reference to the igneous rocks of this area will be found in the remarkable essay by De la Beche in vol. i. of the _Mem. Geol. Surv._ pp. 34, 38.]