ii. BEDDED TUFFS AND LAVAS
During at least the earlier part of the period of the puys, in some districts or from certain vents, such as those of East Fife, Western Midlothian, Eastern Linlithgowshire, Northern Ayrshire, Heads of Ayr and Lower Eskdale, only fine tuff seems to have been thrown out, which we now find intercalated among the surrounding strata. These eruptions, neither so vigorous nor so long-continued as those of the plateaux, never gave forth such thick and widespread sheets of fragmentary materials as those associated with the plateaux in East Lothian and the north-east of Ayrshire. A single discharge of ashes seems in many cases to have been the sole achievement of one of those little volcanoes; at least only one thin band of tuff may be discoverable to mark its activity.
The tuff of these solitary bands is seldom coarse in texture. It usually consists of the ordinary dull green paste, with dust and lapilli of basic pumice. The local variations in the tuffs of the puys generally arise mainly from differences in the composition, size and numbers of the included ejected blocks. Generally the most abundant stones are pieces of different diabases, or basalts; then come fragments from the surrounding Carboniferous strata, from older tuffs and rarely from rocks of much deeper-seated origin.
Now and then the eruptions of tuff have consisted of extremely fine volcanic dust, which, mingling with water, took the form of a compact mudstone, as in the case of the Houston Marls ([p. 423]), which remind one of a volcanic mud. But in most localities the discharge of tuff, though for a time it may have completely obscured the ordinary contemporaneous sedimentation, was intermittent, so that in the intervals between successive showers of detritus, the deposition of non-volcanic sediment went on as usual. Hence it is that bands of tuff, whether they lie among lavas or among sedimentary formations, are apt to contain interstratifications of sandstone, shale, limestone or other detrital deposit, and to pass insensibly into these. The extremely gentle gradation from volcanic into non-volcanic sediment, and the occasional reappearance of thin partings of tuff bring vividly before the mind the slow dying out of volcanic energy among the Carboniferous lagoons.
Fig. 150.—Section in old quarry, west of Wester Ochiltree, Linlithgowshire. Calciferous Sandstone series.
The comparatively quiet character of the volcanic explosions, and the contemporaneous undisturbed deposition of sediment during the earlier part of the puy period, are exemplified in many sections throughout the areas above enumerated, as will be more fully illustrated in subsequent pages. Two typical examples may suffice for this general statement of the characters of the discharges of tuff in the puy-eruptions. In the Linlithgowshire quarry represented in [Fig. 150], where about ten feet of strata have been exposed, a black shale (1) of the usual carbonaceous character, so common in the Oil-shale series of this region, may be seen at the bottom of the section. It is covered by a bed of nodular bluish-grey tuff (2) containing black shale fragments. A second black shale (3) is succeeded by a second thin band of fine pale yellowish tuff (4). Black shale (5) again supervenes, containing rounded fragments of tuff, perhaps ejected lapilli, and passing up into a layer of tuff (6). It is evident that we have here a continuous deposit of black shale which was three times interrupted by showers of volcanic dust and stones. At the close of the third interruption, the deposition of the shale was renewed and continued, with sufficient slowness to permit of the segregation of thin seams and nodules of clay ironstone round the decomposing organic remains of the muddy bottom (7). A fourth volcanic interlude now took place, and the floor of the water was once more covered with tuff (8). But the old conditions of deposit were immediately afterwards resumed (9); the muddy bottom was abundantly peopled with ostracod crustaceans, while many fishes, whose coprolites have been left in the mud, haunted the locality. At last, however, a much more serious volcanic explosion took place. A coarse agglomeratic tuff (10), with blocks sometimes nearly three feet in diameter, was then thrown out, and overspread the lagoon.[459]
[459] See Geol. Surv. Memoir of Edinburgh, p. 45. These tuffs are further described on pp. 465 et seq.
The second illustration may be taken from the admirable coast-section between Burntisland and Kinghorn, where the number of intercalations of tuff is very great. Besides thicker well-marked bands, successive innumerable thin layers occur there among the associated zones of sedimentary strata which separate the sheets of basalt. The character of these tuff-seams may be inferred from the following details of less than two feet of rock at Pettycur Point:—
| Tuff | 1·5 | inch. |
| Limestone | 0·2 | " |
| Tuff | 0·5 | " |
| Shale | 0·2 | " |
| Tuff | 0·1 | " |
| Shale and tuff | 1·0 | " |
| Shale | 0·2 | " |
| Limestone | 0·5 | " |
| Shale full of volcanic dust | 3·5 | " |
| Shaly limestone | 1·5 | " |
| Laminated tufaceous limestone | 2·0 | " |
| Limestone in thin bands, with thin laminæ of tuff | 0·8 | " |
| Granular tuff | 0·6 | " |
| Argillaceous limestone, with diffused tuff | 0·9 | " |
| Fine granular tuff | 0·7 | " |
| Argillaceous limestone, with diffused tuff | 1·5 | " |
| Laminated limestone | 0·1 | " |
| Limestone, with parting of granular tuff in middle | 0·9 | " |
| Tufaceous shale | 2·0 | " |
| Limestone | 0·4 | " |
| Shaly tuff | 1·25 | " |
| Laminated limestone | 0·1 | " |
| Tuff | 1·2 | " |
| 21·65 | inches. |