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 determined. 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.
WARWICKSHIRE
Some fifty miles to the north-east of the Malvern Hills, in the heart of the rich Midlands, and among the coal-fields and the New Red Sandstone to which these Midlands owe so much of their manufacturing industry and their agricultural fertility, another little tract of Cambrian rocks rises to the surface on the east side of the Warwickshire coal-field between Nuneaton and Atherstone. So unobtrusively do these ancient strata take their place among their younger peers, that their venerable antiquity was for a long time undetected.[125] They were actually regarded as parts of the Carboniferous series, which at first sight they seem to underlie conformably. It was not until 1882 that the mistake was corrected by Professor Lapworth, who proved the rocks to be Cambrian by finding undoubted Upper Cambrian fossils in them.[126] Subsequent investigation enabled him to work out the detailed sequence of these strata. He found that the supposed "Millstone Grit" is a thick-bedded quartzite perhaps 1000 feet in thickness, and resembling the well-known quartzites of the Lickey and Caer Caradoc. The "Coal-shales" proved to be a series (possibly 2000 feet thick) of purple, green, grey and black shales, which from their fossils could be paralleled with the dark shales of the Upper Cambrian series of the Malvern Hills.[127] These shales are immediately overlain by the Coal-measures.
[125] Their antiquity was recognized by Yates as far back as 1825 (Trans. Geol. Soc. 2nd series, vol. ii. p. 261). They had been confounded with "Millstone Grit" and "Coal-shale" by Conybeare and Phillips, and this mistake was adopted on the maps and memoirs of the Geological Survey.
[126] Geol. Mag. (1882), p. 563.
[127] Op. cit. (1886), p. 319.
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.
[128] Op. cit. p. 323.
[129] Op. cit. p. 557.
[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.