v. HOW THE PHYSICAL GEOGRAPHY ASSOCIATED WITH ANCIENT VOLCANOES IS

ASCERTAINED

While the materials erupted from old volcanic vents tell plainly enough their subterranean origin, they may leave us quite in the dark as to the conditions under which they were thrown out at the surface. Yet a careful examination of the strata associated with them may throw much light on the circumstances in which the eruptions took place. Many of the results of such examination will be given in subsequent chapters. I will here submit illustrations of how four different phases of physical geography during former volcanic eruptions may be satisfactorily determined.

1. _Submarine Eruptions._--As by far the largest accessible part of the crust of the earth consists of old marine sediments, it is natural that the volcanic records preserved in that crust should be mainly those of submarine eruptions. That many lavas during the geological past were poured out upon the sea-bottom is plainly shown by the thick beds of marine organisms which they have overspread and which lie above them (Fig. 19). In Central Scotland, for example, sheets of basalt have flowed over a sea-bottom on which thick groves of crinoids, bunches of coral and crowds of sea-shells were living. Not less striking is the evidence supplied by bands of tuff. Around Limerick, for instance, the thick Carboniferous Limestone is interrupted by many thin layers of tuff marking intervals when showers of volcanic dust fell over the sea-bottom, killing off the organisms that lived there. But the limestone that overlies these volcanic intercalations is again crowded with fossils, proving that the crinoids, corals and shells once more spread over the place and flourished as abundantly as ever above the tuff.

The accompanying diagram (Fig. 19) illustrates these statements. At the bottom a thick mass of limestone (_l_) full of crinoids, corals, brachiopods and other marine organisms bears witness to a long time of repose, when the clear sea-water teemed with life. At last a volcanic explosion took place, which threw out the first seam of tuff (_t_). But this was only a transient interruption, for the accumulation of calcareous sediment was immediately resumed, and the next band of limestone was laid down. Thereafter a more prolonged or vigorous eruption ejected a larger mass of dust and stones, which fell over the bottom and prevented the continuation of the limestone. But that the sea still abounded in life is shown by the numerous organisms imbedded in the second stratified band of tuff. At last an access of volcanic vigour gave vent to a stream of slaggy lava, which rolled over the sea-bottom and solidified in the thick sheet of amydaloidal basalt marked B. This outflow was followed by a further discharge of ashes and stones, which, from their absence of stratification, may be supposed to have been the result of a single explosion, or at least to have fallen too rapidly for the marine currents to rearrange them in layers. When the water cleared, the abundant sea-creatures returned, and from their crowded remains limestone once more gathered over the bottom. Yet the volcanic history had not then reached its close, for again there came a discharge of ashes, followed by the outpouring of a second lava, which consolidated as a sheet of columnar basalt (B').

It is not necessary, in order to prove the eruptions to have been submarine, that organic remains should be found in the tuffs or between them. If the volcanic ejections are intercalated among strata which elsewhere can be proved to be marine, their discharge must obviously have taken place under the sea. The vent that discharged them may have raised its head above the sea-level, but its lavas and tuffs were spread out over the adjoining sea-floor.

2. _Lacustrine Eruptions._--The same line of evidence furnishes proof that some volcanoes arose in inland sheets of water. If their products are interstratified among sandstones, gravels and shell-marls, wherein the remains of land-plants, insects and lacustrine shells, are preserved, we may be confident that the eruptions took place in or near to some lake-basin. The older lavas and tuffs of Central France supply an instructive example of such an association. In Britain, the abundant and extensive outpouring of lavas and tuffs during the time of the Lower Old Red Sandstone probably occurred in large lakes. Among the sediments of these bodies of water, interstratified between the volcanic sheets, remains of land-plants are abundant, together with, here and there, those of myriapods washed down from the woodlands, and of many forms of ganoid fishes.

3. _Fluviatile Eruptions._--Volcanoes have sometimes arisen on river-plains or on the edges of valleys and gorges, and have poured out their lavas and discharged their ashes over the channels or alluvial lands of the streams. Volcanic materials, usurping the water-channels, bury or are interstratified with fluviatile sand or shingle, containing perhaps remains of the vegetation or animal life of the surrounding land. There may thus be a constant shifting of the river-courses, and a consequent deposit of fluviatile sediment at many successive levels among the lavas and tuffs. In Fig. 20 some of these changes are indicated in a series of bedded lavas (_l_). The lower part of the diagram shows the dying out of a bed of river gravel (_g_) against the sloping end of a lava-stream, and the sealing up of this intercalation by a fresh outpouring of lava. Higher up in the diagram a section is shown of a gully or ravine which has been cut out of the lavas by a stream, and has become choked up with water-worn detritus. Subsequent outflows of lava have rolled over this channel and sealed it up. Examples of such intercalations of lava with old river deposits, and of the burying of water-courses, will be cited in the account of the Tertiary volcanic plateaux of Britain in Chapter xxxviii.

4. _Terrestrial Eruptions._--That volcanoes in former times broke out on land as well as in water may readily be expected. But it is obvious that the proofs of a terrestrial origin may not be always easy to obtain, for every land-surface is exposed to denudation; and thus the relics of the eruptions of one age may be effaced by the winds, rains, frosts and rivers of the next. In assigning any volcanic group to a terrestrial origin, we may be guided partly by negative evidence, such as the absence of all trace of marine organisms in any of the sedimentary layers associated with the group. But such evidence standing by itself would not be satisfactory or sufficient. If, however, between the sheets of lava there occur occasional depressions, filled with hardened sediment full of land-plants, with possibly traces of insects and other terrestrial organisms, we may with some confidence infer that these silted-up hollows represent pools or lakes that gathered on the surface of the lava-sheets, and into which the vegetation of the surrounding ground was blown or washed. Rain falling on the rugged surface of a lava-field would naturally gather into pools and lakes, as the bottoms of the hollows became "puddled" by the gradual decay of the rock and the washing of fine silt into the crevices of the lava.

Again, it may be expected that prolonged exposure to the air would give rise to disintegration of the lava and to the consequent formation of soil. Terrestrial vegetation would naturally spring up on such soil; trees might take root upon it. Hence, if another lava-flood deluged the surface, the soil and its vegetable mantle would be entombed under the molten rock.

These geological changes are represented diagrammatically in Fig. 21. Two hollows among the lavas are there shown to have been filled with silt, including successive layers of vegetation now converted into coal. One of the soils (_s_) is marked between the lavas, and the charred stump of a tree with its roots still in another layer of soil higher up is seen to have been engulphed in the overlying sheet of melted rock.

Admirable illustrations of this succession of events are to be encountered among the great Tertiary basaltic plateaux which cover so large an area in the north-west of Europe. Not only has no trace of any marine organism been found among their interstratified sedimentary layers, but they have yielded a terrestrial flora which is preserved in hollows between the successive sheets of basalt. A full account of these rocks will be given in Book VIII.