[2] Capt. C. E. Dutton. _Sixth Ann. Rep. U.S. Geol. Survey_, 1884-85.
[3] Dutton, _loc. cit._, chap. iv. p. 165.
[4] _Amer. Jour. Science_, vol. 3., ser. (1871). A beautiful map of this mountain is given in the _Fifth Annual Report, U.S. Geol. Survey_, 1883-84. Plate 44.
[5] Daubeny, _loc. cit._, p. 474.
[6] Gilbert, _Monograph U.S. Geol. Survey_, vol. i. (1890).
[7] Powell, _Exploration of the Colorado River_, p. 177, etc. (1875).
Hayden, _Rep. U.S. Geol. Survey of the Colorado, etc._ (1871-80).
[8] Richthofen, _Natural System of Volcanic Rocks_, Mem. California Acad. Sciences, vol. i. (1868).
[9] Geikie, _Geological Sketches at Home and Abroad_, p. 271 (1882).
[10] Prestwich, _Geology_, vol. i. p. 370, quoting from Richthofen.
[11] The origin of geysers is variously explained; see Prestwich, _Geology_, vol. i. p. 170. They are probably due to heated waters suddenly converted into steam by contact with rock at a high temperature.
CHAPTER III.
VOLCANOES OF NEW ZEALAND.
One other region of volcanic action remains to be noticed before passing on to the consideration of those of less recent age. New Zealand is an island wherein seem to be concentrated all the phenomena of volcanic action of past and present time. Though it is doubtful if the term "active," in its full sense, can be applied to any of the existing craters (with two or three exceptions, such as Tongariro and Whakari Island), we find craters and cones in great numbers in perfectly fresh condition, extensive sheets of trachytic and basaltic lavas, ashes, and agglomerates; lava-floods descending from the ruptured craters of ashes and scoriae; old crater-basins converted into lakes; geysers, hot springs and fumaroles which may be counted by hundreds, and cataracts breaking over barriers of siliceous sinter; and, lastly, lofty volcanic mountains vying in magnitude with Vesuvius and Etna. All these wonderful exhibitions of moribund volcanic action seem to be concentrated in the northern island of Auckland. The southern island, which is the larger, also has its natural attractions, but they are of a different kind; chief of all is the grand range of mountains called, not inappropriately, the "Southern Alps," vying with its European representative in the loftiness of its peaks and the splendour of its snowfields and glaciers, but formed of more ancient and solid rocks than those of the northern island.
(_a._) _Auckland District._--We are indebted to several naturalists for our knowledge of the volcanic regions of New Zealand, but chiefly to Ferdinand von Hochstetter, whose beautiful maps and graphic descriptions leave nothing to be desired.[1] In this work Hochstetter was assisted by Julius Haast and Sir J. Hector. From their account we learn that the Isthmus of Auckland is one of the most remarkable volcanic districts in the world. It is characterised by a large number of extinct cinder-cones, in a greater or less perfect state of preservation, and giving origin to lava-streams which have poured down the sides of the hills on to the plains. Besides these are others formed of stratified tuff, with interior craters, surrounding in mural cliffs eruptive cones of scoriae, ashes, and lapilli; these cones are scattered over the isthmus and shores of Waitemata and Manukau. The tuff cones and craters rise from a floor of Tertiary sandstone and shale, the horizontal strata of which are laid open in the precipitous bluffs of Waitemata and Manukau harbours; they sometimes contain fossil shells of the genera _Pecten_, _Nucula_, _Cardium_, _Turbo_, and _Neritae_. As the volcanic tuff-beds are intermingled with the Upper Tertiary strata, it is inferred that the first outbursts of volcanic forces occurred when the region was still beneath the waters of the ocean. Cross-sections show that the different layers slope both outwards (parallel to the sides) and inwards towards the bottom of the craters. Sometimes these craters have been converted into lakes, as in the case of those of the Eifel; but generally they are dry or have a floor of morass. Of the crater-lakes, those of Kohuora, five in number, are perhaps the most remarkable; and in the case of two of these the central cones of slag appear as islets rising from the surface of the waters. The fresh-water lake Pupuka has a depth of twenty-eight fathoms. To the north of Auckland Harbour rises out of the waters of the Hauraki Gulf the cone of Rangitoto, 920 feet high, the flanks formed of rugged streams of basalt, and the summit crowned by a circular crater of slag and ash, out of the centre of which rises a second cone with the vent of eruption. This is the largest and newest of the Auckland volcanoes, and appears to have been built up by successive outpourings of basaltic lava from the central orifice, after the general elevation of the island.
[Illustration: Fig. 26.--Forms of volcanic tuff cones, with their cross-sections, in the Province of Auckland.--No. 1. Simple tuff cone with central crater; No. 2. Outer tuff cone with interior cinder cone and crater; No. 3. The same with lava-stream issuing from the interior cone.--(After Hochstetter.)]
Before leaving the description of the tuff-cones, which are a peculiar feature in the volcanic phenomena of New Zealand, and are of many forms and varieties, we must refer to that of Mount Wellington (Maunga Rei).
This is a compound volcano, in which the oldest and smallest of the group is a tuff-crater-cone, exhibiting very beautifully the outward slope of its beds. Within this crater arise two cones of cinders, each with small craters. It would appear that after a long interval the larger of the two principal cones, formed of cinders and known as Mount Wellington, burst forth from the southern margin of the older tuff-cone, and, being built up to a height of 850 feet, gradually overspread the sides of its older neighbour. Mount Wellington itself has three craters, and from these large streams of basaltic lava have issued forth in a westerly direction, while a branch entered and partially filled the old tuff-crater to the northwards.
Southwards from Manukau Harbour, and extending a short distance from the coast-line to Taranaki Point, there occurs a plateau of basalt-conglomerate (_Basaltkonglomerat_), with sheets of basaltic lava overspreading the Tertiary strata. These plateau-basalts are intersected by eruptive masses in the form of dykes, but still there are no craters or cones of eruption to be seen; so that we may infer that the sheets, at least, were extruded from fissures in the manner of those of the Colorado or Idaho regions of America. Proceeding still further south into the interior of the island, we here find a lofty plateau of an average elevation of 2,000 feet, interposed between the Tertiary beds of the Upper and Middle Waikato, and formed of trachytic and pitch-stone tuff, amongst which arise old extinct volcanic cones, such as those of Karioi, Pirongia, Kakepuku, Maunga Tautari, Aroha, and many others.
These trachytic lavas would seem to be more ancient than the basaltic, previously described.
(_b._) _Taupo Lake, and surrounding district._--But of all these volcanic districts, none is more remarkable than that surrounding the Taupo Lake, which lies amidst the Tertiary strata of the Upper Waikato Basin. The surface of this lake is 1,250 feet above that of the ocean, and its margin is enclosed within a border of rhyolite and pitchstone--rising into a mass of the same material 1,800 feet high on the eastern side. The form of the lake does not suggest that it is itself the crater of a volcano, but rather that it was originated by subsidence. On all sides, however, trachytic cones arise, of which the most remarkable group lies to the south of the lake, just in front of the two giant trachytic cones, the loftiest in New Zealand, one called Tongariro, rising about 6,500 feet, and the other Ruapahu, which attains an elevation of over 9,000 feet, with the summit capped by snow. These two lofty cones, standing side by side, are supposed by the Maoris to be the husband and wife to whom were born the group of smaller cones above referred to as occupying the southern shore of Taupo Lake. The volcano of Tongariro may still be considered as in a state of activity, as its two craters (Ngauruhoe and Ketetahi) constantly emit steam, and several solfataras break out on its flanks.[2]
(_c._) _Roto Mahana._--In a northerly direction from Tongariro, and distant from the coast by a few miles, lies in the Bay of Plenty the second of the active volcanoes of New Zealand, the volcanic island of Whakari (White Island), from the crater of which are constantly erupted vast masses of steam clouds. The distance between these two active craters is 120 nautical miles; and along the tract joining them steam-jets and geysers issue forth from the deep fissures through which the lava sheets have formerly been extruded. Numerous lakes also occupy the larger cavities in the ground; and hot-springs, steam-fumaroles and solfataras burst out in great numbers along the banks of the Roto Mahana Lake and the Kaiwaka River by which it is drained. Amongst such eruptions of hot-water and steam we might expect the formation of siliceous sinter, and the deposition of sulphur and other minerals; nor will our expectations be disappointed. For here we have the wonderful terraces of siliceous sinter deposited by the waters entering Roto Mahana as they descend from the numerous hot-springs or pools near its margin. All travellers concur in describing these terraces as the most wonderful of all the wonders of the Lake district of New Zealand--so great is their extent, and so rich and varied is their colouring.
The beautiful map of Roto Mahana on an enlarged scale by Hochstetter shows no fewer than ten large sinter terraces descending towards the margin of this lake, besides several mud-springs, fumaroles, and solfataras. But the largest and most celebrated of all the sinter terraces has within the last few years been buried from view beneath a flood of volcanic trass, or mud, an event which was as unexpected as it was unwelcome. In May, 1887, the mountain of Tarawera, which rises to the north-east of Roto Mahana, and on the line of eruption above described, suddenly burst forth into violent activity, covering the country for miles around with clouds of ashes, and, pouring down torrents of mud, completely enveloped the beautiful terrace of sinter which had previously been one of the wonders of New Zealand. By the same eruption several human beings were entombed, and their residences destroyed.
The waters of Roto Mahana, together with the hot-springs and fountains are fed from rain, and from the waters of Taupo Lake, which, sinking through fissures in the ground, come in contact with the interior heated matter, and thus steam at high temperature and pressure is generated.[3]
(_d._) _Moribund condition of New Zealand Volcanoes._--From what has been said, it will be inferred that in the case of New Zealand, as in those of Auvergne, the Eifel and Lower Rhine, Arabia, and Western America, we have an example of a region wherein the volcanic forces are well-nigh spent, but in which they were in a state of extraordinary activity throughout the later Tertiary, down to the commencement of the present epoch. In most of these cases the secondary phenomena of vulcanicity are abundantly manifest; but the great exhibitions of igneous action, when the plains were devastated by sheets of lava, and cones and craters were piled up through hundreds and thousands of feet, have for the present, at least, passed away.
[1] _Geol.-topographischer Atlas von Neu-Seeland_, von Dr. Ferd. von Hochstetter und Dr. A. Petermann. Gotha: Justus Perthes (1863). Also _New Zealand_, trans. by E. Sauter, Stuttgart (1867).
[2] Tongariro was visited in 1851 by Mr. H. Dyson, who describes the eruption of steam.
[3] Mr. Froude figures and describes the two terraces, the "White" and "Pink," in _Oceana_, 2nd edition, pp. 285-291.
PART IV.
TERTIARY VOLCANIC DISTRICTS OF THE BRITISH ISLES.
CHAPTER I.
ANTRIM.
It is an easy transition to pass from the consideration of European and other dormant, or extinct, volcanic regions to those of the British Isles, though the volcanic forces may have become in this latter instance quiescent for a somewhat longer period. In all the cases we have been considering, whether those of Central Italy, of the Rhine and Moselle, of Auvergne, or of Syria and Arabia, the cones and craters of eruption are generally present entire, or but slightly modified in form and size by the effects of time. But in the case of the Tertiary volcanic districts of the British Isles this is not so. On the contrary, these more prominent features of vulcanicity over the surface of the ground have been removed by the agents of denudation, and our observations are confined to the phenomena presented by extensive sheets of lava and beds of ash, or the stumps and necks of former vents of eruption, together with dykes of trap by which the plateau-lavas are everywhere traversed or intersected.
The volcanic region of the British Isles extends at intervals from the North-east of Ireland through the Island of Mull and adjoining districts on the mainland of Morvern and Ardnamurchan into the Isle of Skye, and comprises several smaller islets; the whole being included in the general name of the Inner Hebrides. It is doubtful if the volcanic lavas of Co. Antrim were ever physically connected with those of the west of Scotland, though they may be considered as contemporary with them; and in all cases the existing tracts of volcanic rock are mere fragments of those originally formed by the extrusion of lavas from vents of eruption. In addition to these, there are large areas of volcanic rock overspread by the waters of the ocean.
(_a._) _Geological Age._--The British volcanic eruptions now under consideration are all later than the Cretaceous period. Throughout Antrim, and in parts of Mull, the lavas are found resting on highly eroded faces either of the Upper Chalk (Fig. 27), or, where it has been altogether denuded away, on still older Mesozoic strata. From the relations of the basaltic sheets of Antrim to the Upper Chalk, it is clear that the latter formation, after its deposition beneath the waters of the Cretaceous seas, was elevated into dry land and exposed to a long period of subaerial erosion before the first sheets of lava invaded the surface of the ground. We are, therefore, tolerably safe in considering the first eruptions to belong to the Tertiary period; but the evidence, derived as it is exclusively from plant remains, is somewhat conflicting as to the precise epoch to which the lavas and beds of tuff containing the plant-remains are to be referred. The probabilities appear to be that they are of Miocene age; and if so, the trachytic lavas, which in Antrim are older than those containing plants, may be referred to a still earlier epoch--namely, that of the Eocene.[1] As plant remains are not very distinctive, the question regarding the exact time of the first volcanic eruptions will probably remain for ever undecided; but we are not likely to be much in error if we consider the entire volcanic period to range from the close of the Eocene to that of the Miocene; by far the greater mass of the volcanic rocks being referable to the latter epoch.
In describing the British volcanic districts it will be most convenient to deal with them in three divisions--viz., those of Antrim, Mull, and Skye, commencing with Antrim.[2]
(_b._) _Volcanic Area._--The great sheets of basalt and other volcanic products of the North-east of Ireland overspread almost the whole of the County Antrim, and adjoining districts of Londonderry and Tyrone, breaking off in a fine mural escarpment along the northern shore of Belfast Lough and the sea coast throughout the whole of its range from Larne Harbour to Lough Foyle; the only direction in which these features subside into the general level of the country being around the shores of Lough Neagh. Several outliers of the volcanic sheets are to be found at intervals around the great central plateau; such as those of Rathlin Island, Island Magee, and Scrabo Hill in Co. Down. The area of the basaltic plateau may be roughly estimated at 2,000 square miles.
[Illustration: Fig. 27.--"The White Rocks," Portrush, Co. Antrim, showing the plateau-basalt resting on an eroded surface of the Upper Chalk, with bands of flint.--(From a photograph.)]
The truncated edges of this marginal escarpment rising to levels of 1,000 to 1,260 feet, as in the case of Benevenagh in Co. Derry, and 1,825 feet at Mullaghmore, attest an originally greatly more extended range of the basaltic sheets; and it is not improbable that at the close of the Miocene epoch they extended right across the present estuary of Lough Foyle to the flanks of the mountains of Inishowen in Donegal in one direction, and to those of Slieve Croob in the other. In the direction of Scotland the promontories of Kintyre and Islay doubtless formed a part of the original margin. Throughout this vast area the volcanic lavas rest on an exceedingly varied rocky floor, both as regards composition and geological age. (See Fig. 28.) Throughout the central, southern, eastern, and northern parts of their extent, the Chalk formation may be considered to form this floor; but in the direction of Armagh and Tyrone, towards the southwestern margin, the basaltic sheets are found resting indiscriminately on Silurian, Carboniferous, and Triassic strata. The general relations of the plateau-basalts to the underlying formations show, that at the close of the Cretaceous period there had been considerable terrestrial disturbances and great subaerial denudation, resulting in some cases in the complete destruction of the whole of the Cretaceous strata, before the lava floods were poured out; owing to which, these latter are found resting on formations of older date than the Cretaceous.[3]
[Illustration: Fig. 28.--Section across the volcanic plateau of Antrim, from the Highlands of Inishowen, Co. Donegal, on the N.W., to Belfast Lough on the S.E., to show the relations of the volcanic rocks to the older formations.--B. Basaltic sheets breaking off in high escarpments; T. Trachyte porphyry of Tardree mountain rising from below the newer plateau-basalts; C. Upper Chalk with flints; N.R. New Red marl and sandstone (Trias); M. Metamorphic beds of quartzite, various schists and crystalline limestone; F. Large fault.]
[1] Mr. J. Starkie Gardner, from a recent comparison of the plant-remains of Antrim and Mull, concludes that "that they might belong to any age between the beginning and the end of the warmer Eocene period; and that they cannot be of earlier, and are unlikely to be of later, date."--_Trans. Palaeont. Soc._, vol. xxxvii. (1883).
[2] Having dealt with this district rather fully in _The Physical Geology and Geography of Ireland_ (Edit. 1891, p. 81), and also in my Presidential Address (Section C.) at the meeting of the British Association, 1874, a brief review of the subject will be sufficient here, the reader being referred to the former treatises for fuller details. The following should also be consulted: Gen. Portlock, _Geology of Londonderry and Tyrone_ (1843); Sir A. Geikie, "History of Volcanic Action during the Tertiary Period in the British Isles," _Trans. Roy.
Soc. Edinburgh_, 1888; and the _Descriptive Memoirs_ of the Geological Survey relating to this tract of country.
[3] Owing to the superposition of the basaltic masses on beds of chalk throughout a long line of coast, we are presented with the curious spectacle of the whitest rocks in nature overlain by the blackest, as may be seen in the cliffs at Larne, Glenarm, Kinbane and Portrush. (See Fig. 27.)
CHAPTER II.