Map of the Caldera of Palma and the great ravine, called "Barranco de las Angustias." From the Survey of Capt. Vidal, R. N., 1837. Scale, two geographical miles to an inch.

tom of the Caldera, and 4000 above the sea, and situated at the precise limit of two geological formations presently to be mentioned. This col also occurs at the level where, in other parts of the Caldera, the vertical precipices join the talus-like, rocky slope, covered with pines. The other or principal entrance by which the Caldera is drained is the great ravine or barranco, as it is called (see b, b', fig. 696), which extends from the southwestern extremity of the Caldera to the sea, a distance of 44 geographical miles, in which space the water of the torrent falls about 1500 feet.

This sketch (fig. 697) was taken by Von Buch from a point at sea not visited by us, but we saw enough to convince us that several lateral cones ought to have been introduced on the great slope to the left, besides numerous deep furrows radiating from near the summit to the sea (see the map, fig. 696). The sea does not enter the great Barranco, as might be inferred from this sketch.

View of the Isle of Palma, and of the entrance into the central cavity or Caldera
From Von Buch's "Canary Islands."

The annexed section (fig. 698) passes through the island from Santa Cruz de Palma to Briera Point, or from southeast to northwest (see map, p. 628). It has been drawn up on a true scale of heights and horizontal distances from the observations of Mr. Hartung and my own.

Fig. 698.

S.P. a

Section of the Island of Palma, from Point Briera, on the northwest, to Santa Cruz de Palma,

on the southeast. See map, fig. 695, p. 628.

a, b. The Caldera (height of a, 6000 feet).

c. Commencement of steeper dip.

d. Santa Cruz de Palma or Tedote.

e. Lateral cone, 3940 feet above the sea (Vidal's Map).

f. Briera Point.

g. One of several outliers of the upper formation in centre of Caldera.
S. P. Half-buried cone and crater of San Pedro.

The lavas are seen to be slightly inclined near the sea at Santa Cruz, where we observed them flowing round the cone of San Pedro, which they have more than half buried without entering the crater. On starting from the same part of the seacoast, and ascending the deep Barranco de la Madera, we saw just below c the basaltic lavas dipping at an angle of 5 degrees, there being no dikes in that region. Farther up, where the dikes were still scarce, the dip of the beds increases to 10 and 15 degrees, and they become still steeper as they approach the Caldera at b, where dikes abound.

The section (fig. 699) is at right angles to the preceding, and cuts through the cone in the direction of the great Barranco, or from northeast to southwest.

The lowest of the two slanting lines, m, i, descending from the Caldera to the sea along the bottom of the Barranco, represents the present bed of the torrent; the upper line, k, l, the height at which beds of gravel, elevated high above the present river-channel, are visible in detached patches, shown by dotted spaces at k, and to the southwest of it, on the same slope. These, and the continuous stratified

Fig. 699.

SECTION OF THE ISLAND OF PALMA, FROM NORTHEAST TO SOUTHWEST.

f

This section passes through the Caldera and the Barranco de las Angustias, and is drawn up on a true scale of height and horizontal distance, from the observations of C. Lyell and G. Hartung. 1854. (See Map, p. 628.)

a. Barlovento Point, see map, fig. 695, p. 628.

b. One of several cones, S.S.E. from Barlovento Point.

c. Pico de la Cruz, 7780 feet high, forming part of the northern boundary of the
Caldera.
c, d. The Caldera.

d. The summit of the mountain called Alejanado, 6210 feet high, forming the
southern wall of the Caldera.

e. The Cumbrecito, or higher opening into the Caldera.

f. Pico de Cedro, 7470 feet high; the highest point on the eastern margin of the Caldera.

g. Lateral cone on the flanks of Alejanado.

h. Cone of Argual.

i. Cliff of Tazacorte.

k, . Old inclined water-line, marked by upper limit of gravel or conglomerate. m, i. Level of the river or torrent of the Barranco de las Angustias.

The stronger lines in this diagram express that part which alone falls into the line of section; the fainter lines that portion of the eastern circuit of the Caldera which is in perspective, and could be seen by a spectator standing on the west side.

gravel and conglomerate lower down at 7 and i, are newer than all the volcanic rocks seen in this section.

The upper volcanic formation, to be described in the sequel, is traversed by numerous dikes, which could not be expressed on this small scale. The vertical lines in the lower formation represent a few of the perpendicular dikes which abound there. Countless others, inclined and tortuous, are found penetrating the same rocks. The five outliers of somewhat pyramidal shape, at the bottom of the Caldera (on each side of m), agree in structure and composition with the upper formation, and may have subsided into their present position, if the Caldera was caused by engulfment, or may have slid down in the form of land-slips, if the cavity be attributed chiefly to aqueous

erosion.

In the description above given of the section (fig. 699), the cliffs which wall in the Caldera are spoken of as consisting of two formations. Of these the uppermost alone gives rise to vertical precipices, from the base of which the lower descends in steep slopes, which, although they have the external aspect of taluses, are not in fact made up of broken materials, or of ruins detached from the higher rocks, but consist of rocks in place. Both formations are of volcanic origin, but they differ in composition and structure. In the upper, the beds consist of agglomerate, scoriæ, lapilli, and lava, chiefly basaltic, the whole dipping outwards, as if from the axis of the original cone, at angles varying from 10 to 28 degrees. The solid lavas do not constitute more than a fourth of the entire mass, and are divided into beds of very variable thickness, some scoriaceous and vesicular, others more compact, and even in some cases rudely columnar. All these more stony masses are seen to thin out and come to an end wherever they can be traced horizontally for a distance of half or a quarter of a mile, and usually sooner. Coarse breccias or agglomerates predominate in the lower part, as if the commencement of the second series of rocks marked an era of violent gaseous explosions. Single beds of this aggregate of angular stones and scoriæ attain a thickness of from 200 to 300 feet. They are united together by a paste of volcanic dust or spongiform scoria.

At one point on the right side of the great Barranco, near its exit from the Caldera, we observed in the boundary precipice a lofty column of amorphous and scoriaceous rock in which the red or rustcolored scoriæ are as twisted and ropy as any to be seen on the slopes of Vesuvius; seeming to imply that there was here an ancient vent or channel of discharge subsequently buried under the products of newer eruptions. Countless dikes, more or less vertical, consisting chiefly of basaltic lava, traverse the walls of the Caldera, some of them terminating upwards, but a great number reaching the very crest of the ridge, and therefore having been posterior in origin to the whole precipice.

We could not discover in any one of the fallen masses of agglomcrate which strewed the base of the cliffs a single pebble or water-worn

fragment. Each imbedded stone is either angular, or, if globular, consists of scoriæ more or less spongy, and evidently not owing its shape to attrition. It would be impossible to account for the absence of water-worn pebbles if the coarse breccia in question had been spread by aqueous agency over a horizontal area coextensive with the Caldera and the volcanic rocks which surround it. The only cause known to us capable of dispersing such heavy fragments, some of them 3, 4, or or 6 feet in diameter, without blunting their edges, is the power of steam, unless indeed we could suppose that ice had coöperated with water in motion; and the interference of ice cannot be suspected in this latitude (28° 40'), especially as I looked in vain for signs of glacial action here and in the other mountainous regions of the Canary Islands.

The lower formation of the Caldera is, as before stated, equally of igneous origin. It differs in its prevailing color from the upper, exhibiting a tea-green and in parts a light yellow tint, instead of the usual brown, lead-colored, or reddish hues of basalt and its associated scoriæ. Beds of a light greenish tuff are common, together with trachytic and greenstone rocks, the whole so reticulated by dikes, some vertical, others oblique, others tortuous, that we found it impossible to determine the general dip of the beds, although at the head of the great gorge or Barranco they certainly dip outwards, or to the south, as stated by Von Buch. But in following the section down the same ravine, where the mountain called Alejanado (d, figs. pp. 628 and 631) is cut through, and where the rocks of the lower formation are very crystalline, we found what is not alluded to by the Prussian geologist, that the beds exposed to view in cliffs 1500 feet high have an anticlinal arrangement, exhibiting first a southerly and then a northerly dip at angles varying from 20 to 40 degrees (see section, fig. 699, at k). Hence we may presume that the older strata must have undergone great movements before the upper formation was superimposed. No organic remains having been discovered in the older series, we cannot positively decide whether it was of subaerial or submarine origin. We can only affirm that it has been produced by successive eruptions, chiefly of felspathic lavas and tuffs. Many beds which probably consisted at first of soft tuffs have been much hardened by the contact of dikes and apparently much altered by other plutonic influence, so that they have acquired a semicrystalline and almost metamorphic char

acter.

The existence of so great a mass of volcanic rocks of ancient date on the exact site of an equally vast accumulation of comparatively modern lavas and scorice is peculiarly worthy of notice as a general phenomenon observed in very different parts of the globe. It proves that, notwithstanding the fact in the past history of volcanoes that one region after another has been for ages and has then ceased to be the chief theatre of igneous action, still the activity of subterranean heat may often be persistent for more than one geological period in