Fig. 462.
When trap dikes were described in the preceding chapter, they were shown to be more modern than all the strata which they traverse. A basaltic dike at Quarrington Hill, near Durham, passes through coal-measures, the strata of which are inclined, and shifted so that those on the north side of the dike are 24 feet above the level of the corresponding beds on the south side (see section, [fig. 463.]). But the horizontal beds of overlying Red Sandstone and Magnesian Limestone are not cut through by the dike. Now here the coal-measures were not only deposited, but had subsequently been disturbed, fissured, and shifted, before the fluid trap now forming the dike was introduced into a rent. It is also clear that some of the upper edges of the coal strata, together with the upper part of the dike, had been subsequently removed by denudation before the lower New Red Sandstone and Magnesian Limestone were superimposed. Even in this case, however, although the date of the volcanic eruption is brought within narrow limits, it cannot be defined with precision; it may have happened either at the close of the Carboniferous period, or early in that of the Lower New Red Sandstone, or between these two periods, when the state of the animate creation and the physical geography of Europe were gradually changing from the type of the Carboniferous era to that of the Permian.
Fig. 463.
Section at Quarrington Hill, east of Durham. (Sedgwick.)
- a. Magnesian Limestone (Permian).
- b. Lower New Red Sandstone.
- c. Coal strata.
The test of age by superposition is strictly applicable to all stratified volcanic tuffs, according to the rules already explained in the case of other sedimentary deposits. (See [p. 96.])
Test of age by organic remains.—We have seen how, in the vicinity of active volcanos, scoriæ, pumice, fine sand, and fragments of rock are thrown up into the air, and then showered down upon the land, or into neighbouring lakes or seas. In the tuffs so formed shells, corals, or any other durable organic bodies which may happen to be strewed over the bottom of a lake or sea will be imbedded, and thus continue as permanent memorials of the geological period when the volcanic eruption occurred. Tufaceous strata thus formed in the neighbourhood of Vesuvius, Etna, Stromboli, and other volcanos now active in islands or near the sea, may give information of the relative age of these tuffs at some remote future period when the fires of these mountains are extinguished. By such evidence we can distinctly establish the coincidence in age of volcanic rocks, and the different primary, secondary, and tertiary fossiliferous strata already considered.