[42] Student's Manual of Geology, 2d edit. (1862), p. 523.
In Millersdale the authors of the Geological Survey Memoir on North Derbyshire observed that a band of toadstone about 100 feet thick showed six distinct divisions, which they were disposed to regard as marking so many separate beds.[43] In Tideswell Dale, on the west side of the valley, immediately to the south of the old toadstone quarry, two bands of toadstone are seen to be separated by a few yards of limestone.
[43] Op. cit. p. 19.
(b) Lavas with Tuffs.—It will probably be found that in many, if not in most cases, the outflow of lava was preceded, accompanied or followed by fragmental discharges. As far back as 1861, Jukes noticed that a toadstone band, about 50 feet thick, near Buxton consisted of two solid beds of lava "with beds of purple and green ash, greatly decomposed into clay, both above and below each bed and between the two."[44]
[44] Op. cit. p. 523>.
Fig. 183.—Section at lime-kiln, south of Viaduct, Millersdale Station.
An interesting section, showing this intercalation of the two kinds of material is exposed at the lime-kilns beyond the southern end of the railway viaduct at Millersdale Station. Over a mass of solid blue limestone (1 in [Fig. 183]) lies a band of bright yellow and brown clay (2), varying from six inches to two feet in thickness. This may be compared with the clay found above the limestone at Peak Forest ([Fig. 181]). But it is probably a layer of highly decomposed tuff. It is succeeded by a thin band of greenish limestone (3) containing an admixture of fine volcanic detritus, and partially cut out by an irregular bed, four to eight feet thick, of a highly slaggy, greenish, decomposing, spheroidal and amygdaloidal diabase (4). This unmistakable lava-sheet is followed by a bed of green granular tuff (5), which in some places reaches a thickness of three feet, but rapidly dies out. Over a space several yards in breadth, the succeeding strata are concealed, and the next visible rock is a dark, compact dolerite which weathers spheroidally (6).
(c) Tuffs without Lavas.—Mr. Bemrose has shown that some of the bands of toadstone consist entirely of bedded tuff. In these cases, so far as the present visible outcrops allow us to judge, no outflow of lava accompanied the eruption of fragmentary materials. But that the ejection of these materials was not the result of a sudden spasmodic explosion, but of a continued series of discharges varying in duration and intensity, is indicated by the well-bedded character of the tuff and the alternation of finer and coarser layers. Large blocks of lava, two feet or more in diameter, may mark some of the more vigorous paroxysms of the vents, while the usual fine granular nature of the tuff may point to the prevailing uniformity and less violent character of the eruptions. Bands of tuff 70 feet or more in thickness, without the intercalation of any limestone or other non-volcanic intercalation, point to episodes of such continued volcanic activity that the ordinary sedimentation of the sea-bottom was interrupted, or at least masked, by the abundant fall of dust and stones.
One of the best exposures of such intercalations of bedded tuffs was pointed out to me by Mr. Bemrose, immediately to the east of the village of Litton. The matrix is crowded with the usual minutely vesicular glassy lapilli, and encloses fragments of diabase of all sizes, up to blocks more than a foot in diameter. The rock is well stratified, and the layers of coarse and fine detritus pass beneath a group of limestone beds. The actual junction is concealed under the roadway, but only two or three feet of rock cannot be seen. The lowest visible layer of limestone is nodular and contains decayed bluish fragments which may be volcanic lapilli. Immediately above the lower limestones the calcareous bands become richly fossiliferous. Some of their layers consist mainly of large bunches of coral; others are crowded with cup-corals, or are made up mainly of crinoids with abundant brachiopods, polyzoa, lamellibranchs, gasteropods and occasional fish-teeth. This remarkable profusion of marine life is interesting inasmuch as it succeeds immediately the band of volcanic ash.