70 
THE CAMBRIAN VOLCANOES 
BOOK HI 
antiquity of which is proved by the position of the Cambrian fossiliferous 
strata that overlie them. But these strata themselves include certain 
igneous rocks whicli point to a recrudescence of eruptive energy in a far 
later geologictd period. 
Nearly half a century has passed away since John Phillips men- 
tioned the intercalation of igneous rocks in the series of strata which is 
now classed as Upper Cambrian in the Malvera Hills. Since that date 
liardly anything has been added to the information which lie collected. 
The existence of a group of rocks of such high antiquity, asserted to be of 
truly volcanic origin, and the precise horizon of winch 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 
Fig. 45. — Section across the Cambrian formations of the Malvern Hills, showing the position of the 
intercalated igneous rocks After Phillips. 
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 cfinspicuous 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 aniygdaloidal and prismatic structures among 
them.^ The position of these eruptive rocks is shown in Fig. 45. 
These rocks were afterwards observed and described by Hr. Holl, who 
found what he considered to be four true lava sheets interstratilied in the 
Hollybush Sandstones. He noted the intercalation of “ numerous beds of 
volcanic ash, grit and lava ” in the black shales.^ 
So far as I am aware, no more recent account of these rocks has been 
published. Their true stratigraphical and petrographical relations require 
to be more precisely deterinined. If they are really contemporaneous lavas, 
they point to volcanic eruptions at the time when the middle division of 
the Cambrian system was being deposited. If, on the other hand, they 
should prove to be intrusive, they would indicate probable volcanic activity 
in this part of England at some time later than the middle of the Cambrian 
period. 
' Mem. Geul. Survey, vol. ii. part i. pp. 52, 55 ; also Horizontal Sections of the Geol. Survey, 
Sheet 1-3, No. 8, and Sheet 15. Reference to the igneous rocks of this area will he found in the 
remarkable essay by De la Beclie in vol. i. of the Mem. Geol. Sun. pp. 34, .38. 
^ Quart. Journ. Geol. Soc. vol. xxi. (1865), pp. 87-91. 
