B. Tuffs
The fragmental rocks connected with the puy-eruptions form a well-marked group, easily distinguishable, for the most part, from the tuffs of the plateaux. They vary from exceedingly fine compacted dust or volcanic mud, through various stages of increasing coarseness of texture, to basalt-conglomerates and tumultuous agglomerates.
The fragmentary material found in the necks of the puys is generally an agglomerate of a dull dirty-green colour. The matrix ranges from a fine compact volcanic mud to a thoroughly granular detritus, and sometimes shows a spheroidal concentric structure in weathering. In this matrix the lapilli are distributed with great irregularity and in constantly varying proportions. They consist in large measure of a pale yellowish-green, sometimes pale grey, very basic, finely vesicular, devitrified glass, which is generally much decomposed and cuts easily with the knife. This highly basic substance is a kind of palagonite. So minute are its vesicles that under the microscope a thin slice may present a delicate lace-like network of connected walls, the palagonite occupying much less space than the vesicles. The material has been a finely frothed-up pumice.
Besides this generally distributed basic pumice, the stones in the agglomerate of the necks likewise include fragments of older volcanic grits or tuffs, blocks of basalt or diabase, as well as pieces of the Carboniferous strata of the district, especially shale, sandstone and limestone. Not infrequently also, they comprise angular blocks of fossil wood.
The materials which fill the necks are generally much coarser than those that form intercalated beds. But while in numerous cases huge blocks of basalt and large masses of sandstone, shale, limestone, ironstone or other strata may be seen wrapped up in a matrix of coarse basalt-tuff, in not a few instances the material in the necks may be observed to consist of a tuff quite as fine as that of the interstratified bands. Such necks appear to mark the sites of tuff-cones where only fine ashes and lapilli were ejected, and where, after sometimes a brief and feeble period of activity, the orifice became extinct.
The bedded tuffs interstratified with the ordinary Carboniferous strata do not essentially differ in composition from the material of the necks. They are basalt- (diabase-) tuffs and basalt- (diabase-) conglomerates, usually dull green in colour and granular in texture, the lapilli consisting in great measure of various more or less decayed basalts, but containing the same highly vesicular basic glass or pumice above referred to. They are mainly to be distinguished by their conspicuous stratification, and especially by their rapid alternations of coarser and finer material, by the intercalation of shales, limestones, sandstones or ironstones in them, and by the insensible gradations by which they pass both vertically and laterally into ordinary sediments. Occasional large blocks or bombs, indicating some paroxysm of explosion, may be observed even among the finer tuffs, shales and other strata, which round the sides of these masses have had their layers bent down by the fall of heavy blocks.[449] Many of the bedded tuffs contain fossils, such as crinoids, corals, brachiopods, fish-teeth or macerated fragments of land-plants. Coal-seams also are occasionally interstratified among them.
[449] Ante, [p. 36], and Figs. [15] and [151]. See also Geol. Mag. i. (1864), p. 22; Trans. Roy. Soc. Edin. vol. xxix. (1879) p. 515.
Of the finer kinds, the best example is furnished by a remarkable group of "green and red marls" which lie above a seam of coal (Houston Coal) in the Calciferous Sandstones of West Lothian.[450] These strata, which differ much from any of the rocks with which they are associated, are exceedingly fine in grain, dull sage-green and brownish or chocolate-red in colour, not well laminated like the shales, but breaking under the influence of weathering into angular fragments, sometimes with a conchoidal fracture. They look like indurated mud. Mr. H. M. Cadell, who has recently re-examined them in connection with a revision of the Geological Survey Map (Sheet 32) has found them passing into ordinary granular tuff.
[450] Memoir on Sheet 32 Geol. Surv. Scotland (1861), p. 42. The stratigraphical position of these "Houston Marls," as they are locally called, is indicated in [Fig. 155].
Palagonitic-tuff is of frequent occurrence. It is met with in the Firth of Forth district,[451] and Mr. Watts has detected fragments of palagonite among the tuffs of the Limerick basin.
[451] Trans. Roy. Soc. Edin. vol. xxix. (1819) p. 515.
CHAPTER XXVII
GEOLOGICAL STRUCTURE OF THE CARBONIFEROUS PUYS OF SCOTLAND
1. Vents: Relation of the Necks to the Rocks through which they rise—Evidence of the probable Subærial Character of some of the Cones or Puys of Tuff—Entombment of the Volcanic Cones and their Relation to the Superficial Ejections. 2. Bedded Tuffs and Lavas—Effects of Subsequent Dislocations. 3. Sills, Bosses and Dykes.
The puy-type of volcanic hill differs widely in one respect from those which we have hitherto been considering. In the earlier epochs of volcanism within the British area, it is the masses of material discharged from the vent, rather than the vents themselves which arrest attention. Indeed, so copiously have these masses been erupted that the vents are often buried, or their positions have been rendered doubtful, by the uprise in and around them of sills and bosses of molten rock. But among the Carboniferous puys the vent is often the only record that remains of the volcanic activity. In some cases we know that it never ejected any igneous material to the surface. In others, though it may be filled with volcanic agglomerate or tuff, there is no record of any shower of such detritus having been discharged from it. In yet a third class of examples, we see that lava rose in the vent, but no evidence remains as to whether or not it ever flowed out above ground. Other cases occur where beds of lava or of tuff, or of both together, have been intercalated in a group of strata, but with no trace now visible of the vent from which they came. The most complete chronicle, preserving at once a record of the outflow of lava, of the showering forth of ashes and bombs, and of the necks that mark the vents of eruption, is only to be found in some of the districts.
I shall therefore, in the present instance, reverse the order of arrangement followed in the previous chapters, and treat first of the vents, then of the materials emitted from them, and lastly of the sills and dykes.