While the external forms of these Velay necks recall with singular vividness the features of many more ancient necks in Britain, an examination of the internal structure of some of them affords some further interesting points of resemblance. The slabs into which, by means of weathering along the joints, the rock is apt to split up are sometimes arranged with a general dip outwards from the centre of the hill, so that their flat surfaces roughly coincide with the hillslopes. In other cases the peculiar platy structure, so characteristic of phonolite, is disposed vertically or dips at a steep angle into the hill, so that the edges of the slabs are presented to the declivities, which consequently become more abrupt and rugged.
Though none of the volcanic series in Auvergne or the Velay is so acid in composition as the more acid members of the Tertiary volcanic series of Britain, the manner in which the trachytes and phonolites of the French region make their appearance presents some suggestive analogies to that of the corresponding rocks in this country. We see that they were erupted long after the outpouring of extensive basaltic plateaux, that they belonged to successive epochs of volcanic activity, that they were protruded in a pasty condition to the surface, where, more or less covered with fragmentary ejections, they terminated in dome-shaped hills or spread out to a limited distance around the vents, and lastly, that they were succeeded by a still later series of more basic eruptions, which completed the long volcanic history. We shall see in the following pages how closely the various stages in this complex record of volcanic activity may be paralleled in the geological records of Tertiary time in Britain.[385]
[385] The phonolite necks of Bohemia, which form so prominent a feature in the Tertiary geology of that country, might likewise be cited here in illustration of the acid domes and bosses of the British Isles.
CHAPTER XLVI
TYPES OF STRUCTURE IN THE ACID ROCKS—BOSSES
Returning now to the consideration of the acid rocks as these manifest themselves in the volcanic areas of Britain, I would remark that three distinct types of structure may be noted among them, viz. (1) bosses, (2) sills or intrusive sheets, (3) veins and dykes. These types, as above remarked, belong entirely to the underground operations of volcanism, for though the rhyolitic fragments in the tuffs and agglomerates of the plateaux prove that acid lavas existed near the surface, no undoubted case of superficial lava belonging to the acid series has yet been observed.[386]
[386] The rhyolites of Tardree in Antrim have recently been claimed by Professor Cole as true lavas grouped round an eruptive vent. For reasons to be given in the next chapter I regard them as intrusive masses, though they may not improbably have been connected with streams of lava now entirely removed.
The bosses of acid material in the British Tertiary volcanic series are irregular protrusions, varying in size from knobs only a few square yards in area up to huge masses many square miles in extent, and comprising groups of lofty hills. As a rule, their outlines are markedly irregular. Beneath the surface they plunge down almost vertically through the rocks which they traverse, but in not a few instances their boundaries are inclined to the horizon, so that the contiguous rocks seem to rest against them, and sometimes lie in outliers on their sides and summits. From the margins of these bosses apophyses are given off into the surrounding rocks, sometimes only rarely and at wide intervals, in other places in prodigious numbers. Sometimes the acid material has been injected in thousands of veins and minute threads, which completely enclose fragments of the surrounding rock.
The rock of which the bosses consist is generally granophyric in texture, passing on the one hand, particularly in the central parts, into granite, and on the other, and especially towards the margin, into various more compact felsitic varieties, and sometimes exhibiting along the outer edge more or less developed spherulitic and flow-structures.
Decided contact metamorphism is traceable round the bosses, but is by no means uniform even in the same rock, some parts being highly altered, while others, exposed apparently to the same influences, have undergone little change. The most marked examples of this metamorphism are those in which the Cambrian limestone of Skye has been converted into a pure white saccharoid marble. But the most interesting to the student of volcanic action are those where the altered rocks are older parts of the volcanic series. As the bosses of each volcanic area offer distinctive peculiarities they will here be described geographically.