Part 62 (1/2)

[389-A] See Princ. of Geol., _Index_, ”Graham Island,” ”Nyoe,”

”Conglomerates, volcanic,” &c.

[390-A] MacCulloch, West. Isl., vol. ii. p. 487.

[390-B] Syst. of Geol., vol. ii. p. 114.

[390-C] Ibid.

[392-A] See Principles, chaps. xxiv-xxvii.

[393-A] See Principles, chaps. xxvi. and x.x.x.; 8th ed. p. 397-475.

[394-A] See Principles of Geol. ch. xxiv. (8th ed. p. 355.).

[394-B] See Lyell on Craters of Denudation, Quart. Geol. Journ.

vol. vi. p. 232.

CHAPTER x.x.x.

ON THE DIFFERENT AGES OF THE VOLCANIC ROCKS.

Tests of relative age of volcanic rocks--Test by superposition and intrusion--Dike of Quarrington Hill, Durham--Test by alteration of rocks in contact--Test by organic remains--Test of age by mineral character--Test by included fragments--Volcanic rocks of the Post-Pliocene period--Basalt of Bay of Trezza in Sicily--Post-Pliocene volcanic rocks near Naples--Dikes of Somma--Igneous formations of the Newer Pliocene period--Val di Noto in Sicily.

Having referred the sedimentary strata to a long succession of geological periods, we have next to consider how far the volcanic formations can be cla.s.sed in a similar chronological order. The tests of relative age in this cla.s.s of rocks are four:--1st, superposition and intrusion, with or without alteration of the rocks in contact; 2d, organic remains; 3d, mineral character; 4th, included fragments of older rocks.

[Ill.u.s.tration: Fig. 461. Cross section.]

_Tests by superposition, &c._--If a volcanic rock rests upon an aqueous deposit, the former must be the newest of the two, but the like rule does not hold good where the aqueous formation rests upon the volcanic, for melted matter, rising from below, may penetrate a sedimentary ma.s.s without reaching the surface, or may be forced in conformably between two strata, as _b_ at D in the annexed figure (fig. 461.), after which it may cool down and consolidate. Superposition, therefore, is not of the same value as a test of age in the unstratified volcanic rocks as in fossiliferous formations. We can only rely implicitly on this test where the volcanic rocks are contemporaneous, not where they are intrusive. Now they are said to be contemporaneous if produced by volcanic action, which was going on simultaneously with the deposition of the strata with which they are a.s.sociated. Thus in the section at D (fig. 461.), we may perhaps ascertain that the trap _b_ flowed over the fossiliferous bed _c_, and that, after its consolidation, _a_ was deposited upon it, _a_ and _c_ both belonging to the same geological period. But if the stratum _a_ be altered by _b_ at the point of contact, we must then conclude the trap to have been intrusive, or if, in pursuing _b_ for some distance, we find at length that it cuts through the stratum _a_, and then overlies it as at E.

We may, however, be easily deceived in supposing a volcanic rock to be intrusive, when in reality it is contemporaneous; for a sheet of lava, as it spreads over the bottom of the sea, cannot rest everywhere upon the same stratum, either because these have been denuded, or because, if newly thrown down, they thin out in certain places, thus allowing the lava to cross their edges. Besides, the heavy igneous fluid will often, as it moves along, cut a channel into beds of soft mud and sand. Suppose the submarine lava F to have come in contact in this manner with the strata _a_, _b_, _c_, and that after its consolidation, the strata _d_, _e_, are thrown down in a nearly horizontal position, yet so as to lie unconformably to F, the appearance of subsequent intrusion will here be complete, although the trap is in fact contemporaneous. We must not, therefore, hastily infer that the rock F is intrusive, unless we find the strata _d_ or _e_ to have been altered at their junction, as if by heat.

[Ill.u.s.tration: Fig. 462. Cross section.]

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, pa.s.ses through coal-measures, the strata of which are inclined, and s.h.i.+fted 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 s.h.i.+fted, 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.

[Ill.u.s.tration: 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, scoriae, 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 sh.e.l.ls, 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.

The tuffs now alluded to are not exclusively marine, but include, in some places, freshwater sh.e.l.ls; in others, the bones of terrestrial quadrupeds.

The diversity of organic remains in formations of this nature is perfectly intelligible, if we reflect on the wide dispersion of ejected matter during late eruptions, such as that of the volcano of Coseguina, in the province of Nicaragua, January 19. 1835. Hot cinders and fine scoriae were then cast up to a vast height, and covered the ground as they fell to the depth of more than 10 feet, and for a distance of 8 leagues from the crater in a southerly direction. Birds, cattle, and wild animals were scorched to death in great numbers, and buried in these ashes. Some volcanic dust fell at Chiapa, upwards of 1200 miles to windward of the volcano, a striking proof of a counter current in the upper region of the atmosphere; and some on Jamaica, about 700 miles distant to the north-east. In the sea, also, at the distance of 1100 miles from the point of eruption, Captain Eden of the Conway sailed 40 miles through floating pumice, among which were some pieces of considerable size.[399-A]