The Malvern Hills.—It was clearly shown by Dr. Holl, in 1865, (Quar. Jour. Geol. Soc., Vol. xxi.) that the central portion or axis of this chain of hills consists of a great series of true crystalline schists, among which the prevailing types are hornblendic and micaceous gneiss, hornblende-schist, mica-schist, and a quartzo-felspathic rock, all of which are more or less distinctly foliated. There is, however, great variety in the relative proportions of the constituents; in many places either the hornblende or the mica are nearly, or even quite absent; the felspar and quartz then form the mass of the rock. In the quarries on the east side of the North Hill, beautiful examples of hornblende and felspar rock are abundant; they contain more or less quartz, with a little mica, and occasionally pass by insensible gradations from a well-marked gneissic structure into a coarsely crystalline mass in which foliation is no longer apparent. These latter are, however, exceptional cases, and there can, I think, be no doubt that we have here, in the Midlands, a considerable exposure of the oldest type of foliated crystalline schists, or as they are now frequently called Pre-Cambrian or Archæan rocks. The rocks of the North Hill are probably the oldest; they have been much disturbed, and are generally the most coarsely crystalline of the series. The more basic portions have suffered a considerable amount of alteration; the secondary constituents, chlorite and epidote, become abundant, and occasionally the hornblende has been completely replaced by the latter mineral; the rock then becomes an epidote schist of a pale-yellowish colour. In Swinyards Hill there is a micaceous gneiss with garnets, and in Raggedstone Hill there are interesting varieties of contorted mica-schist containing a large proportion of quartz. Dykes of intrusive rocks occur in nearly all the hills, but become far more numerous towards the north; they are very uniform in appearance, and are probably altered dolerites or diorites; very few specimens have been examined, and they belong to the former group. In the North Hill there are some masses of true diorite, but their relation to the hornblendic gneisses with which they are associated has not been clearly established.
Mount Sorrel Granite.—The granite is of two varieties, red and grey, the difference being due to the fact, that in the red masses, the partially decomposed felspar has been coloured by ferric oxide. The rock is a hornblendic granite, the constituents being quartz, felspar, biotite, hornblende and titanite, with magnetite and a little apatite. The felspars are orthoclase and plagioclase; the former is much decomposed, while the plagioclase frequently remains clear, and exhibits well its twin striation. Biotite was originally abundant, but is very frequently replaced by a clear green substance, which is strongly dichroic, the two colours being grass-green, and clear yellow. The hornblende has been greatly decomposed; clear crystals are, however, not uncommon, and exhibit the usual optical characters of the mineral. The products of alteration are chlorite and epidote. The titanite appears in reddish-brown grains, but is not very abundant. In 1879, the writer discovered the junction of the granite with the sedimentary rocks, and proved that the former was intrusive (Geol. Mag. dec. ii., Vol. vi., p. 181). The junction occurs in Brazil Wood, where there is a small quarry, in which granite veins may be seen to penetrate the strata in various directions; a large mass of granite is also within a few yards. Some of the phenomena of contact metamorphism may here be readily studied, the granite having converted the slate into a crystalline micaceous schist, quite similar in character to those produced under like conditions round the granite in Cornwall and elsewhere. Small garnets occur in the altered slates, and also in the granite close to the junction.
The Charnwood Syenites.—The Syenites and other igneous rocks of Charnwood, have been described by Messrs. Hill and Bonney (Quar. Jour. Geol. Soc. Vol. xxxiv., p. 199.) The original constituents are felspar, hornblende, quartz, apatite, ilmenite, magnetite and titanite. The felspar is of two kinds, orthoclase and plagioclase, the former is very turbid and decomposed, while the plagioclase is clear, and retains its usual optical characters. The curious intercrystallization of quartz and felspar, known as micropegmatite, is common in the masses of syenite near Groby; the best examples, however, have been found by the writer in the Markfield rock, where it appears to form a ground-mass in which the larger crystals of orthoclase and plagioclase are enclosed. A small portion of the hornblende is still characteristic, but the greater portion appears in various stages of decomposition; the alteration products being chlorite and epidote. Titanite is by no means rare, and occurs in well formed twin crystals.
Diorites of Atherstone and Nuneaton.—A careful examination of many specimens collected by the writer from the various masses marked in the map 63 S.W. of the Geological Survey shows clearly that they are diorites, the characteristic constituents being hornblende and a triclinic felspar; these minerals, together with magnetite and apatite are invariably present, and in addition, a little orthoclase is seldom absent. The best specimen examined is from a quarry near Marston Jabet; it is a fine-grained rock, similar to a basalt in external appearance, but contains numerous crystals of hornblende and plagioclase which are generally quite unaltered. The clear brown crystals of hornblende are unusually well developed, and afford excellent opportunities of examining their crystallographic and optical characters. There are also present many grains of magnetite, and a few needles of apatite. The only product of alteration is a little calcite in the spaces between the crystals. Other specimens from the same quarry will perhaps give a better idea of the general character of the rocks of the district, and they possess a special interest, as they afford unusually good examples of successive stages of alteration. In one specimen the constituents are well preserved, the plagioclase is clear and exhibits its characteristic twin striation; the hornblende is, for the most part, unaltered, but is much fissured, and occasionally contains so many cavities that the crystals are little more than skeletons. A ground-mass in which the constituents were set has been highly altered, and now consists of a fine granular substance, partly serpentinous, with here and there a little calcite. A second example is quite similar in texture to the last; the felspar, still recognisable as triclinic, has been partially converted into a grey turbid substance. The hornblende occurs in various stages of alteration; some crystals are but slightly attacked, while others are to a considerable extent replaced by a pale green serpentinous substance. The alteration has followed the cleavage-lines and fractures, while the numerous cavities just mentioned are also filled by the same substance. In a single slice there may be seen almost every degree of change from a slight marginal erosion to a mere skeleton of the original. Of the latter, however, some little is always left; and whether the alteration be little or great, the crystalline forms are perfectly preserved. In a third specimen the alteration has proceeded still further, the whole of the hornblende crystals having been completely converted into pale green pseudomorphs; they were originally large and well developed, and their forms are still perfectly sharp and distinct. The felspar is here quite turbid and opaque, and the interstitial ground-mass is represented by calcite.
Purley Park, near Atherstone.—The rock here contains, in addition to the usual brown hornblende, many crystals and grains of clear yellowish augite, and several pseudomorphs after olivine. The augite crystals exhibit the usual forms, some being twins. The pseudomorphs after olivine are quite similar to others observed in certain highly altered dolerites, they consist of calcite or calcite and viridite; they are numerous, and are generally larger than the crystals of augite or hornblende.
Quarry close to Atherstone.—This rock also contains both augite and hornblende; and lastly, in the railway cutting at Chilvers Coton, several interesting varieties of diorite may be found.
The Rhyolites of the Wrekin near Wellington, Shropshire.—In the large quarry in Lawrence Hill, at the north end of the Wrekin, and at Lea Rock on the Shrewsbury Road, are to be found some of the most beautiful varieties of ancient volcanic glassy rocks hitherto discovered in Britain. The rocks in their present condition do not look like glass, owing to a process of devitrification, which they have evidently undergone. There can, however, be no room for doubt as to their original vitreous condition, for they exhibit, under the microscope, certain peculiar perlitic and spherulitic varieties of structure, associated with characteristic forms of microliths, which are found only in the pitchstone and obsidian varieties of volcanic glass. In the quarry in Lawrence Hill, thick beds of volcanic ashes and agglomerates are to be seen dipping towards the north at a high angle, and an examination of this hill and the Wrekin shows that they both consist of a series of stratified ashes alternating with several flows of rhyolite. One of the ash beds contains numerous spheroidal blocks of thoroughly characteristic varieties of glassy rocks; they have not yet been described, but it may here be stated that, in addition to many typical varieties of known rhyolites, there are also included among them some of the rarer glassy kinds described by Zirkel in his Petrography of the 40th Parallel. Beautiful examples of the spherulitic and perlitic varieties may be found at Lea Rock, and have been described by the writer in the Quar. Jour. Geol. Soc. Vol. xxxiii., p. 449. It has been shown by Dr. Callaway, that all these rhyolites are of Pre-Cambrian age; we have here, therefore, the clearest proof that, during very early geological periods volcanic action was of the same kind, and produced the same results, as in more recent times.
South Staffordshire.—The igneous rocks of the South Staffordshire coalfield belong to the basic series, and are, without exception, dolerites or basalts, the latter being merely a fine grained variety of the former. They have been intruded among the coal measures and shales, and are frequently found in an excellent state of preservation. The original mineral constituents are crystals, or crystalline grains of triclinic felspar, augite, olivine, magnetite, ilmenite, and apatite. These minerals are very frequently quite unaltered, with the exception of the olivine, which is often partly converted into serpentine; this is the pale green substance seen along the cracks, and around the sides of the grains; generally, however, the decomposition has been continued until the formation of complete serpentinous pseudomorphs after olivine has been the result. The Hailstone Hill, near Rowley Church, is the best locality for varieties in texture and composition, as also for contemporaneous veins. In the large quarry there is a very coarsely crystalline variety containing large flat plates of ilmenite, and here also may be found some light-coloured veins in which orthoclase is the predominant felspar. The writer has also found in the same quarry vessicular and amygdaloidal varieties of the rock. It need scarcely be mentioned that, minute details of structure and composition can only be studied in thin slices under the microscope. Rocks of similar character to the above occur in the following localities:—Pouk Hill, near Walsall; Titterston Clee Hill (sheet 55, N.W.), Knowl Hill, near Kinlet (sheet 55, N.E.), and Swinnerton Park, eight miles N.E. of Stafford. For fuller descriptions of these rocks, see Allport, Quart. Jour. Geol. Soc., Vol. xxx., p. 529.