the sea. This is the character of the low land extending from the mountains of Kualua round the north point of the island, as far as the bay of Waialua.

The plain of Eva rises by a gradual acclivity from the low land of Waialua on the north, and of Eva on the south, to the estimated height of 200 to 400 feet above the sea. It is of inconsiderable extent; upon a rough estimate, about forty square miles. It is barren, destitute of wood, but thinly inhabited, and the uniformity of its surface is hardly relieved by the ravines of water courses with which it is frequently intersected, and which render travelling across it tedious and fatiguing.

Not an unimportant feature in the low land of Eva are the lagunes at the mouth of the Pearl River. These are formed by breaks in the coral reefs, which form nearly a continuous line all round the island, or rather several parallel lines, one behind the other.

There yet remains to be noticed a very remarkable feature in the physiognomy of Oahu, and one of the first to strike the eye of a stranger, from their accumulation at the eastern extremity of the island. These are the isolated hills observed rising either from the maritime low land, or from the midst of the hilly country. In general form they exactly resemble some of the extinct craters of central France, and are doubtless of the same character. They are all open at top, and their outer walls are furrowed by lava streams. They are generally of a brownish colour externally. I counted no less than five in sailing along the east end of the island, including that of Diamond Point; Punchbowl Hill, just behind the Honoruru, is a well marked one. The salt lake of Aliapaakai, about four miles to the west of Honoruru, is of the same character, and is elevated only a few feet above the level of the sea, from which it is not more than onefourth of a mile distant. According to report it is 40-50 fathoms deep, in form nearly oval, and about one-fourth of a mile in its shortest diameter from east to west. Formerly it yielded large quantities of salt, which was dug out in solid masses from the bottom; but this source of wealth has been stopped by its inundation preventing all access to the salt by such means as the islanders possess. The north-east margin of the crateriform hollow is the highest, and is connected with a small tract of hilly country, extending here up to the mountains.

These isolated crateriform hills only represent, on a small scale, and from their being more detached, with all their characters better defined, the larger crateriform peaks of the two chains, which being aggregated together in a linear form, lose at least the outline of two of their sides. Such is the idea I have formed from an attentive consideration of the paris at the head of the ravines; they are only the inner walls of the crater, which has poured forth the two ridges, or lava streams bounding the ravine.

The total want of any dome-shaped summits, is a remarkable feature in the mountains of Oahu, and one by which they differ materially from Hawaii, where this form is very common, and it is even reported that you can ride to the very summit of Mouna Roa.

In consequence of the peculiar form of the principal mountain valleys of Oahu, there are hardly any rivers, but only mountain streams. The only one which deserves the name, is that which discharges itself into the sea at Waialua, on the north-west side of the island. It does not derive its chief supply of water from either of the mountain chains, but from the plain of Eva, and the hilly tract Koolauloa (plain ?) between which it forms a sort of natural boundary. The length of its course is about ten miles.

General Sketch of Formations.

These may be viewed as consisting of two great classes, the volcanic and the coralline. The first far exceeds the second in quantity, and forms the whole of the interior of the island, as far as it was open to my inspection. The latter is confined to the reefs skirting the sea-shore, or to the cliffs immediately bordering upon it. The former is now on the decrease, from the disintegrating effects of the weather not being counterbalanced by a counteracting formative force. The latter is upon the increase, from the incessant labours of its numerous live inhabitants. The coralline is a recent formation, superimposed upon the previously formed volcanic hills, unless we adopt the hypothesis of part of this limestone mass having been upraised from the bed of the ocean, by the heaving up of the volcanic mass, which is strengthened by the compact and ancient appearance of some of the coral cliffs above Kahuku, at the north point of the island.

Basaltic Lava.-The volcanic formation might be viewed as consisting of two parts,-the more compact and basaltiform, and the scoriform; but these, although in the extreme forming very distinct appearances, pass into each other by insensible gradations; the black basaltic compact rocks are generally seen at a small elevation above the level of the sea, as on the sea-shore to the west of Waihea, on the north side of the island, and at the entrance of the valley of Anuanu. The more porous and scoriform rocks generally form the higher grounds, such as the lateral ridges descending from the central chains. But this last rule has many noted exceptions, one of the most striking of which is, that, at Waimea, on the north-east coast, where scoriæ almost as cellular as pumice are situate nearly at the level of the sea, and under more compact rocks of a porphyritic character, having a dark-blue basis, with imbedded crystals of glassy felspar and olivine, associated in the same hand specimen.

Even the blackest and most compact rocks, when closely examined, are very different from the basalt of the secondary rocks. They have all a certain degree of porosity, however small, and present angular interstices in their mass, giving the idea of their having been formed by agglutination from a semiviscid mass. The following is the series of these rocks that I collected in the order of their similarity to basalt, Nos. 13, 1, 35, 23, 12, 48, 49, 11; 40, 33.* The two last form the bond of connection with the next series, the scoriform. The first is undoubtedly the nearest to basalt, both in its external characters and order of superposition, being from a bed on the sea-shore, near Waihea, which runs into the centre of the island. It will be observed, too, that the nearer in the above order these rocks approach No. 13, the darker is their tint, and the freer they are of imbedded minerals. As we recede, the tint becomes more grey, and we have numerous crystals of glassy felspar, of small size, and grains of olivine, giving to the rock a porphyritic character.

Amygdaloidal Lava.-The next series of the volcanic rocks are the amygdaloidal, under which may be comprehended specimens No. 18, 17, 8, 20, 28, 36, 48, 19, 10, 22, 3, 9. The last connects them with the next series, the scoriform; the amygdaloidal cavities are generally empty, seldom full; their inner

* These numbers refer to the specimens sent home with the sketch, and now deposited in the Royal Museum of the University of Edinburgh.

parietes are often lined, as in No. 28, with a greenish matter. In all these specimens, with the exception of No. 17, the cavities present the idea of having been formed by the extrication of gaseous matter, in a viscid mass, not of cavities which had been once filled with crystalline matter that had subsequently disappeared. In some, the approach to the scoriform character is very close, in others, scoriform, and nearly compact portions are placed in close juxta-position in the same hand specimen, as in No. 8, 9, and 10, which shews upon how weak a foundation is built the determination of the age of these rocks, by their degree of compactness simply. Sometimes a rock will be in one portion of the mass compact and basaltiform, while in others closely adjacent, it will be filled with empty amygdaloidal-shaped cavities.

Cellular Lava.-The third series of the volcanic rock are the cellular or scoriform, comprising specimens, Nos. 24, 5, 25, 44, 27, 26, 8, 45, 43, 6, 41, 4, 7, 37, 39, 47, 38. The cells vary in number from a few scattered amygdaloidal-shaped cavities, till the whole mass resembles closely in porosity some kinds of pumice. In some the roughness and unyielding texture of the mass, and the size and regular form of the cells, give to the whole a slaggy appearance. In many and in all the more cellular varieties, the cavities are empty; in others they are filled with crystals of olivine, which are frequently undergoing decomposition, leaving the cavities partially empty. Angular crystals of mica and felspar frequently give to the rocks a porphyritic character; these crystals are, however, always of small size. The mica is frequently very abundant, forming, with its shining hexagonal plates, the only imbedded mineral. This is well marked in the slaggy lava from the coast at Waimea. In some of the more cellular varieties, the laminæ of mica become more elongated, and are laid along the interstices of the cells, which they never cross. The felspar is white, and dull in lustre. In the lavas of one of the ridges descending from Mouna Raala, it forms a very abundant ingredient, giving to the rock a spotted appearance. This felspar diminishes sometimes so much in the size, whilst the number of its crystals increased, that it forms part of the ass of the rock, which, in such cases, is less cellular, but more pact, rough, and trachytic in its fracture. The most comcolour of the basis of these cellular lavas is ash-grey. Ocsionally the tint becomes reddish, distinct from that of the

matter lining the cavities which has become decomposed. At Waimea some of the most slaggy and scoriform lavas that I saw had a brownish-red colour, passing to cochineal red.

Tuffaceous Lava.-The fourth and last series of volcanic rocks that I noticed, are such as may be classed under the denomination of tuffas. These vary in consistence, from the loose and friable reddish tuffa of the pari of Anuanu, to the compact beds alternating with a basaltic rock on the sea-shore near Waihea. These last are full of amygdaloidal cavities, containing radiated zeolites. The specimens comprised in this series are Nos. 2. 15, 16, 14, 50, and 51. They almost invariably form subordinate beds of greater or less extent in some of the preceding kinds of volcanic rocks. One of the largest of these is at the brink of the pari of Anuanu, and to this circumstance of geognostic structure is the existence of this pass owing, which forms so remarkable a feature in the physiognomy of the chain Ronahuanui, and which formerly decided the political fate of these islands. In the bed of a stream, at the base of Raala, I observed interposed between dark coloured compact lava a bed of a substance, which, although in consistence much resembling tuffa, yet was very different from it. Some resemblance it had to the peperino of Italy. In a basis saffron-yellow, streaked with gallstone-yellow, were imbedded dark-green harder crystals, apparently of augite. The basis of these tuffas was earth, resembling much the earth obtained from the decomposition of lava re-agglutinated together into a firm mud. Its colour varied almost in every locality. In some, wood-brown, wood-brown with a yellowish tinge; in others, tile-red. At the pari Anuanu, where it formed so large a mass, it was light orange-red. Such is a brief outline of the leading generalities of the volcanic rocks; let us now sketch, in the same rapid way, the chief characters of the second class of rocks, the

Coralline Formation.-The best situations for judging of the relations of this formation to the volcanic, are on the north-east and north-west shores of the island. The coral cliffs or reefs, when submerged, form a series of terraces elevated one above another, in proceeding inwards to the interior of the island. The line of these ledges, when under water, can be traced by the successive lines of surf that break them in upon

ranges

be

hind each other. The space between the last ledge above high