GEOLOGY are rocks of Pre-Cambrian age present, but if so, they are veined with later intrusions. The Archaean rocks admit of being classed in three systems, according as to whether they are crystalline, volcanic, or sedimentary. The crystalline rocks are the oldest, and are known in the Malvern and contiguous districts as the Malvernian. They form — except for a very small mass — the Malvern Hills, and comprise a haplite, diorites, felsites, and schistose and gneissic rocks. The gneissic and schistose rocks, according to the most generally accepted views, have been formed out of the others. It is improbable that the haplite and diorites formed part of the original crust. Their mineralogical characters, so far as can be judged, indicate that they must have consolidated and crystallized under the pressure of a con- siderable thickness of rock, but of this original crust nothing is definitely known. Above the Malvernian come the Uriconian and Longmyndian Systems. The Uriconian is essentially a volcanic system. Rocks belonging thereto occur on the confines of Herefordshire and Worcestershire, and form the rocky spurs which project from the eastern base of the Herefordshire Beacon into the Severn Valley. They include rhyolites, andesites, basalts, and tuffs, not differing materially from the products of recent volcanoes, and were first identified as Uriconian by Dr. C. Callaway, who paralleled them with the similar rocks of Lilleshall Hill in Shropshire.'^ Since Dr. Callaway first pointed out their true age, they have been studied by Mr. A. H. Green,' Mr. F- Rutley,' Mr. A. Harker,* and Mr. H. D. Acland.' Beds of sedimentary origin compose the Longmyndian System, but there are no exposures of them in Herefordshire. They may be present far below the surface, flanking, but separated by fault-planes from, the Pre-Cambrian ' massif ' of the Malvern Hills, and underlying the whole of the county. Long before the Longmyndian Beds were formed, and before the period of volcanic activity, the Pre-Cambrian haplite and diorites had in many places been changed into gneisses and schists by mechanical stresses of inconceivable intensity set up by crust-pressure. The massive Malvernian rocks were made to shear and slide over each other, and in many places eventually to assume an appearance very suggestive of thinly stratified sedimentary deposits altered by heat. The latter indeed was the view held by the earlier geologists as to the origin of the crystalline schists of the Malvern Hills. For nearly sixty years preceding the promulgation of the present views as to their structure, the Malvern Hills formed the subject of much geo- logical investigation. Leonard Horner described them as a granitic mass intruded into the associated beds,^ and Murchison also regarded them as essentially of igneous origin, but included in the ' igneous ' mass the strata altered by the intrusion. Professor John Phillips thought that the Palaeo- zoic rocks associated with the range had been laid down against a shore-line composed of the Archaean rocks ;* while H. B. HoU held that the Cambrian ' Quar/. Jourti. Geo/. Soc. xxxvi (1880), p. 536. Mbid. li (1895), pp. 1-8. 'Ibid, xliv (1888), p. 740.
- Petrology for Students (1895), pp. 55, 143, &c. ^ Quart. Journ. Geol. Soc. liv (1898), p. 556.
' Trans. Geol. Soc. Ser. i (181 1), p. 281. ' Silurian System (1839), PP- 4' 7 ^^ ^eq. ; Siluria (1854), p. 92. ' Mag. and Journ. Sci. xxi (1842) ; Mem. Geol. Surv. ii, pt. i (1848), pp. 66 and 125-6. 5
