The observer will readily perceive that much, requiring great care, is needed in investigations of this kind, and that, when endeavouring to trace the paths by which such animals may have migrated, and to ascertain the localities from whence, after retreating, they may again have, in part, been dispersed, districts over which no seas have passed, during the lapse of the supposed geological time, are of no slight value. Hence, among other objects of geological interest, the region of extinct volcanos in Central France is important, inasmuch as it seems to have constituted dry land, during a range of time when several animals which once lived on its surface became extinct, among them the mammoth and Rhinoceros tichorhinus. Amid the various notices of the remains of mammals found in situations giving them geological date, may be mentioned that of M. Pomel, wherein he describes an ossiferous fissure in a lava current (near Orbiéres, on the south of Clermont), which had issued from Gravenoire. It was filled with volcanic sand, pulverulent carbonate of lime, and bones which are stated to be the same as those of Coudes and other contemporaneous accumulations, containing the remains of the elephant, Rhinoceros tichorhinus, horse, ox, &c.* Land shells, of species now existing in the district, are mentioned by M. Pomel, as associated with these ossiferous deposits, so that in this region also, as in others of Europe and North America, great mammals have become extinct, while land and fresh-water molluscs, living with them, have continued to exist up to the present time. Paper (Proceedings of the Geol. Society, vol. iv., p. 36, 1843), on the Geological Position of the Mastodon giganteum, and associated Fossil Remains of Bigbone Lick, Kentucky, and other localities in the United States and Canada, he pointed out that "on both sides of the Appalachian chain the fossil shells, whether land or freshwater, accompanying the bones of the mastodons, agree with species of mollusca now inhabiting the same regions." He also concluded that "the extinct quadrupeds, before alluded to in the United States (mastodon, elephant, mylodon, megatherium, and megalonix), lived after the deposition of the northern drift; and consequently the coldness of climate, which probably coincided in date with the transportal of the drift, was not, as some pretend, the cause of their extinction."

* "Bull. de la Soc. de France," tom. xiv., 1842-3. A very instructive lecture was given by Sir Charles Lycll, at the Royal Institution of Great Britain, on this region in 1847, an account of which appeared in the Athenæum of the time. He especially called attention to changes which its mammals had undergone, as shown by the osseous remains preserved in the alluvium associated with volcanic accumulations, no flood or return of the ocean having disturbed the surface."

CHAPTER XVII.

VOLCANOS AND THEIR PRODUCTS.-HEIGHT ABOVE THE SEA.-CRATERS OF ELEVATION AND ERUPTION.-FOSSILIFEROUS VOLCANIC TUFT BEDS. SEVERAL CRATERS ON ONE FISSURE.-VOLCANIC VAPOURS AND GASES.VOLCANIC SUBLIMATIONS. MOLTEN VOLCANIC PRODUCTS.-FLOW OF LAVA STREAMS.-VESICULAR LAVA.-VOLCANIC CONES.-COTOPAXI.-VOLCANOS OF HAWAII.-EFFECTS OF LAVA ON TREES.

up

DISTRIBUTED over various portions of the earth's surface, as well in high southern and northern latitudes, as in temperate and tropical regions; at points in the ocean far distant from main masses of dry land, as well as upon the latter themselves, free communications are effected between the interior of our planet and its atmospheric covering, through which molten rock, cinders, and ashes are ejected. That great heat, if not the primary, is at least a chief secondary cause by which these mineral substances are thus heaved, is rendered evident by the high temperature of the bodies thrown out. The molten rock flows as a viscous fluid, and retains its high temperature for a long succession of years; and mineral substances are volatilized, which, we learn from our laboratories and furnaces, are only raised to that state by great heat. At the same time that these mineral bodies are ejected, vapours and gases, of a certain marked character, are expelled, so that by carefully combining the mode of occurrence of the various products, with the composition of the substances themselves, an observer, by the aid of sound chemistry and physics, may hope so to direct his inquiries as to obtain a fair insight into the causes and effects of volcanic action.

As regards altitude above the level of the sea, volcanic products are accumulated at various heights above that level, doubtless also forming the bases of many volcanos beneath it on the floor of the ocean. The most elevated of known volcanos constitute such an insignificant fraction of the earth's radius, that variations in height do not appear to offer any great aid in ascertaining the causes of

by 11, portion of sijent and tilted beds of dis-
been, a gd, of similar volcanic

ill
motion, an in fig. 114.
Brion of a better kind would be expected, should the ashes,
and moben mutter have been accumulated both beneath and

el, the action of the breakers denuding the general
mahitane istine is would be afforded. Thus,
Ipired level, the original dome or cone-shaped
mda (fig. 117), though covered, for a time, by a mass of

g, the usult of a high state of activity in the volcano, my fly home visible, and afford the information sought. In the evidence of another kind may be obtained, ang the acculation from simple volcanic eruption, by mie domicin, as shown in fig. 114. In both sections it is

demic action not ceasing, conical accumulations me to be formed inside a crateriform cavity, more or be spent by water, clifs all round facing an active volcanic

Tnberenen these fourable circumstances, the observer should
ply put cution. The facts presented to him may require no
Bink empation and chostication; for in such localities, more
ly, he has to consider how far the relative levels of the sea
addy have remained the same since the various accumula-
bem have been effected. Let it be supposed, for
in, the detects organic remains in beds surrounding
sin of water, in which the volcanic island still vomits
Sich is gases and products. Should the deposits g and h
1) be of the more recent geological times, commonly
by the presence of the remains of molluses, not much, if at
thing those still existing in the vicinity, and should
emposition of the including beds not be decisive on
the subject may not be so clear. By reference to the
114, it will be seen that if the line, de, representing
thest level of the sea, be raised, and, consequently, the whole
of moks, including the supporting deposits, a cb, relatively
de the layers now above the sea, being then below it,
ms may have lived upon and amid these layers while they

ely constituted the sea-bottom, as upon any other sea-
hans, and as many molluses must now do around volcanic islands.
There is no dificulty in considering that, during a long lapse of
time, breaker actin aided in the re-arrangement of many sub-
saces, including animal remains, on the subageous slopes of vol-

he angle of the beds varying according to obvious conditions. ange in the relative levels of the sea and land, which the , as he pursues his researches, will find to have been so t, and often so considerable, that should raise the general g. 114), so that de be the line of sea level, would expose es of these fossiliferous beds facing the interior. And it be borne in mind, that in many localities calcareous beds, en limestone, may become mingled with such deposits during ubmarine accumulation.

en studying the fractures and contortions of rocks, as well on mall as the large scale, there will be frequent occasion to k, as will be more particularly shown hereafter, the mixture xures and fissures, and the extension of the one into the The subjoined example (fig. 115) of the termination of a

a

re and flexure, occurring amid the slightly-inclined beds of ar Lyme Regis, Dorset, may aid in illustrating a point of nterest connected with the present subject, namely, that in re marked instances adduced of "craters of elevation," a rable break or outlet is often found on one side. The plan 15) shows an alternation of the thin-bedded limestone of the Dorsetshire with shale, the whole broken through by a crack, he continuation of one where there is dislocation producing ent on the sides, and which terminates in a boss at b, with hat diverging small cracks. The interior is composed of ne, round which shale, covering it, is exposed by the pearprotrusion, outside which is another limestone bed, c c c, outwards from the central portion, b, the whole taking a rizontal character towards a, where, for a certain length, e surface is merely broken by a fissure. With proper esistances employed, a like disposition of parts could be the large scale.

the subjoined plan (fig. 116), such a state of things rought about on the large scale, and volcanic forces had led to find vent at different points, there may be d

Y

volcanic action, though certain of its effects may thereby be somewhat modified, especially when volcanos rise into the regions of perpetual snow. Cotopaxi, the cone of which rises, in the Andes, 12 leagues S.S.E. from Quito, to the height of somewhat more than 19,000 feet above the sea, forms but an insignificant part of the radius of the earth, not constituting so much as 33 miles of that radius, or about 1 th of it.*

With respect to the kind of openings through which the gaseous and mineral substances are vomited forth, there has existed much difference of opinion. While some geologists infer that the rocks through which the volcanic forces found vent had been so acted upon that they were upraised in a dome-like manner, the gaseous products bursting through the higher part, driving the lighter substances into the atmosphere, if the dome were elevated into it, and raising the viscous molten rock, so that it flowed out of the orifice; others consider that there has been a simple fissure or aperture in the prior-formed rocks through which the volcanic products were propelled, the solid substances accumulating round the vent, so that a deceptive dome-like appearance is presented.

The following sections (figs. 113 and 114) may assist in showing the differences between the "craters of elevation," first brought under notice by M. Von Buch, and so ably illustrated by M. Élie de Beaumont and other geologists, and the "craters of eruption," as they have been termed. Fig. 113 represents a portion of

* Humboldt (Kosmos) refers to the relative height of volcanos as probably of consequence if we should assume their seat of action at an equal depth beneath the general surface of the earth. He refers to eruptions being commonly more rare from lofty than from low volcanos, enumerating the following:-Stromboli, 2318 feet (English); Guacamayo (Province of Quiros), where there are almost daily detonations; Vesuvius, 3876 feet; Etna, 10,870 feet; Peak of Teneriffe, 12,175 feet; and Cotopaxi, 19,070 feet.