But it is the interbedded sheets that possess the chief interest as superficial lava-streams of such venerable antiquity. They present many of the ordinary features of true lava-flows. In particular a slaggy structure may be detected at the bottom of a sheet, the vesicles being here and there lengthened in the direction of flow. Some of the sheets are in part amygdaloidal. The alternation of these sheets with tuffs, evidently derived from lavas of similar character, is another argument in favour of their contemporaneous date. One of the best localities for studying these features lies between Clegyr Foig and the coast, west of Rhosson.
The eruptive rocks thicken towards the south-west, as if the main vents had lain in that direction. There are doubtless intrusive as well as contemporaneously interbedded masses in the rough ground between Pen-maen-melyn and Treginnis. To separate these out would be a most interesting and beautiful piece of mapping for any competent geologist in possession of a good map on a sufficiently large scale.
The interbedded lavas, so far as I have had an opportunity of studying them, appear to present remarkable uniformity of petrographical characters. Megascopically they are dull, fine-grained to compact, sparingly porphyritic, ranging in colour from an epidote-green to dull blackish-green and dark chocolate-brown. Some of them are finely porphyritic from the presence of small glistening surfaces which present the colour and metallic lustre of hæmatite and yield its characteristic streak. Obviously basic rocks, they present, as I have said, a close external resemblance to many of the lavas of the Lower Old Red Sandstone and Carboniferous districts of Scotland. From their chemical composition and microscopic structure they may be most appropriately ranged among the diabases. The analyses of two of the most conspicuous diabases of this class in the district, those of Rhosson (VII.) and Clegyr Foig (VIII.), by Mr. J. S. Grant Wilson, are shown in the following table:—
| SiO2 | Al2O3 | Fe2O3 | FeO | MnO | CaO | MgO | K2O | Na2O | H2O and Loss on Ignition. | Insoluble Residue. | Total. | Specific Gravity. | |
| VII. | 45·92 | 18·16 | 1·18 | 9·27 | 0·19 | 7·19 | 10·07 | 1·78 | 2·12 | 4·22 | 0·04 | 100·14 | 2·96 |
| VIII. | 45·38 | 16·62 | 4·06 | 8·63 | 0·14 | 8·19 | 9·41 | 0·71 | 2·20 | 4·34 | 0·08 | 99·76 | 2·99 |
The two rocks here analyzed, likewise that from the crag south of Castell and that from the cliffs at the southern end of the promontory between Ramsey Sound and Pen-y-foel, show under the microscope a general similarity of composition and structure. They present a variable quantity of a base, which under a ⅕ objective is resolved into ill-defined coalescent globulites and fibre-like bodies, that remain dark when rotated between crossed nicols. In some varieties, as in part of Rhosson Crag, the base is nearly lost in the crowd of crystalline constituents; in others, as in the crag south of Castell, it forms a large part of the whole mass, and may be seen in distinct spaces free from any crystalline particles. Through this base are diffused, in vast numbers, irregularly-shaped grains of augite, seldom showing idiomorphic forms. These grains, or granules, may perhaps average about 0·003 inch in diameter. Plagioclase is generally hardly to be recognized, though here and there a crystal with characteristic twinning may be detected in the base. Magnetite occurs abundantly—its minute octahedra, with their peculiar colour and lustre, being apparent with reflected light on the fresher specimens, though apt to be lost as diffused ferruginous blotches in the more decomposed varieties. But perhaps the most remarkable ingredient is olivine. Red hæmatitic crystals are visible, even to the naked eye, dispersed through the groundmass of the rocks. With a lens these may be observed to be orthorhombic in form, and to be evidently pseudomorphs after some mineral which has been converted chiefly into hæmatite. Such red pseudomorphs are common in Carboniferous and Old Red Sandstone lavas, where in some cases they appear to be after hornblende, and in others after augite, but occasionally are suggestive of olivine, though with no trace of the original substance of that mineral. In the lava associated with the tuffs at the south end of the promontory between Ramsey Sound and Pen-y-foel, however, certain large, well-developed pseudomorphs are undoubtedly after olivine. They have the characteristic contour of that mineral and its peculiar transverse curved and irregular fractures. The average length of these pseudomorphs was found, from the measurement of six examples, to be 0·023 inch, the largest being 0·034, and the smallest 0·014. Seen by transmitted light they present a structureless pale-green material nearly inert in polarized light, round the borders and across fissures in which an opaque substance has been developed, as serpentine and magnetite have been grouped in the familiar alteration of olivine. The opaque material is bright brick-red in reflected light, and is evidently now chiefly oxidized into hæmatite. Every stage may be traced, from orthorhombic forms with the incipient development of transverse fissures filled with iron-oxide, to others of distorted shapes in which the ferruginous matter occupies the whole, or nearly the whole, of the mould of the original crystal.
The rocks now described differ from the Palæozoic andesites or "porphyrites," with which I am acquainted, in their more basic composition, in the less abundance of their microscopic base, in the comparatively inconspicuous development of their felspars of later consolidation, in the absence of large porphyritic felspars of earlier consolidation, in the extraordinary prominence of the granular augite, and in the presence of olivine. In composition and structure they are essentially forms of olivine-diabase.
Fig. 41.—Section showing the interstratification of tuff and conglomerate above Lower Mill, St. David's.
Above the volcanic group of St. David's lies a band of quartz-conglomerate which has been taken by Dr. Hicks to mark the base of the Cambrian system. This rock, though mainly composed of quartz and quartzite, contains fragments of the underlying volcanic rocks. But that it does not mark any decided break in the sedimentation, much less the violent unconformability and vast interval of time which it has been erroneously supposed to do, is well illustrated by the occurrence of bands of tuff, as well as diffused volcanic dust, in the conglomerate and also in the green and red shales and sandstones which conformably overlie it. An example of this intercalation of volcanic material is represented in [Fig. 41]. On the left side vertical layers of fine reddish tuff (a) are succeeded by a band of quartz conglomerate (b) of the usual character. Parallel to this conglomerate comes a band, about six inches thick, of fine tuff (c), followed by ashy sandstone (d), which graduates into conglomerate (e). No more complete evidence could be desired of the perfect inosculation of the conglomerate with the volcanic group. On the coast at Nun's Chapel similar evidence presents itself. The conglomerate there contains some thin seams of tuff, and is intercalated between a series of dull green agglomerates and tuffs and some greenish shales and sandstones with layers of tuff.
Not less striking is the evidence of the contemporaneous eruption of fine volcanic dust in the overlying shales and sandstones.[88] Some of the red shales are full of this material, which here and there is gathered into the thin seams or ribs of which the microscopic characters have already been described. This diffused volcanic detritus marks, no doubt, the enfeebled discharges of fine dust towards the close of the volcanic episode in the Lower Cambrian period at St. David's. It would be difficult to find an instance of a more perfect transition from a series of thoroughly volcanic masses into a series of ordinary mechanical sediments.