Palagonite

“ As will be seen, when travelling to Mosfell, or south-east, we found trachyte on a level with the Esja’s foundation and, when coasting along the western flank, we saw Palagonite sandstone, dyked with trap, and underlying as well as overlying the later igneous formation. The sequence, therefore, appeared to be Palagonite, trachyte, and trap. On the Kollafjörð also there is a line of carbonate of lime running from north-east to south-west, and strongly affecting the water: hence it is judged that Iceland spar may be found there. ”

“ In this matrix the lapilli are distributed with great irregularity and in constantly varying proportions. They consist in large measure of a pale yellowish-green, sometimes pale grey, very basic, finely vesicular, devitrified glass, which is generally much decomposed and cuts easily with the knife. This highly basic substance is a kind of palagonite. So minute are its vesicles that under the microscope a thin slice may present a delicate lace-like network of connected walls, the palagonite occupying much less space than the vesicles. The material has been a finely frothed-up pumice. ”

“ These rocks have also supplied the agglomerates of the locality, and the palagonite-tuffs which are in places extensively represented are evidently in great part derived from vitreous forms of the same rocks. We seem to get nearer to supra-marine eruptions in this region than in most others. The palagonitic-tuffs and agglomerates appear to have rapidly accumulated in shallow water, and there is reason for regarding one exposure of the aphanitic augite-andesites as at all events a shallow-water lava-flow. ”

“ The black-bird (Turdus merula) is sometimes driven to Iceland by southern gales.
[6] Of local specimens we were shown varieties of the Mó-berg (Palagonite tuff) , especially from the Seljadalr, which feels soft, between chalk and steatite, some white or dull yellow, acted upon by acids; others brown and black. Palagonite conglomerate with large pieces of felspar. Blue compact basalt from Kjallarnes, with and without drusic cavities; hexagonal basalt; reniform pebbles of the same material. Jaspers, red, yellow, and green, from the north, the latter containing copper. ”

“ They admit of being examined in thin sections; and their true nature is at times so much disguised that I have taken them at first for aphanitic basic andesites. In the slide they display a few scattered tests of foraminifera of pelagic habit in a matrix composed of the fine debris of palagonite and of basic rocks, together with fragments of plagioclase and pyroxene. Most of the material is very fine, and the size of the largest mineral fragments does not exceed ·2 mm. ”

“ The bomb-rock is a semi-vitreous basaltic andesite. It displays microporphyritic plagioclase in a ground-mass formed mainly of a smoky, almost isotropic glass, in which numbers of felspar microliths (·1 mm.) are developed, the augite being but slightly differentiated. Scattered about in the glass are little irregular patches, or “lakelets,” of residual magma composed of a yellowish feebly refractive material that I cannot distinguish from palagonite. ”

“ Coarse and fine palagonite-tuffs and agglomerate-tuffs of the same character are exposed on the surface from the commencement of the ascent up to 1,850 feet; but they are displayed much more extensively in the stream-course than in the soil-covered upper slopes.
The tuffs are grey except when hydrated, when they turn yellowish-brown. Some of them contain lime, as much at times as 10 or 12 per cent.; whilst others possess little or none. Tests of foraminifera are not infrequently inclosed, even as high as 1,850 feet. ”

“ Coarse Zeolitic Palagonite-tuffs These deposits represent coarse kinds of the submarine tuffs of basic glass, in which the palagonitic change is far advanced, and where zeolites and at times secondary calcite have been produced in abundance as a result of the alteration. They present themselves in the mass as mottled grey rocks which when examined in thin sections are seen to be composed in great part of fragments of more or less palagonitised vacuolar basic glass, whilst zeolites are extensively developed in numerous irregular cavities and in the interspaces. ”

“ The appearance is that of a spherulitic rather than an oolitic structure.
"A slice [6658 A] shows the general mass of the rock to be of an extremely finely divided but coherent substance of brown colour, which can scarcely be other than a fine volcanic dust, composed of minute particles of basic glass or 'palagonite' compacted together. Scattered through this are fragments of crystals recognizable as triclinic and perhaps monoclinic felspars, green hornblende, augite, olivine (?) , and magnetite, usually quite fresh. ”

“ The western features are, as a rule, broad, with shallow sag: here, according to some, [79] was deposited the Surtarbrand [80] or lignite, and, like the driftwood of Kerguelen Island, it escaped incineration by subsequent eruptions from causes analogous to the operation of charcoal burning. The northern firths are long and deeply indented, and the eastern are sharp and narrow, encased in walls of Palagonite, trap, and basalt. ”

“ There are streams of lava down to the very shore of the sea, but no pumice. Some of the trachytic lavas are said to abound with crystals of albite. It is important to examinexxxi whether or not this is oligoclase, as on Teneriffe, Popocatepetl, and Chimborazo; or labradorite, as on Etna and Stromboli. Palagonite, exactly similar to that of Iceland or in Italy, was discovered by Bunsen in the specimens of tufa from Chatham Island, one of the Gallipagos. ”

“ Their size is ·3 to ·6 mm., and they apparently represent the minute amygdules of palagonite that occupy the vacuoles in an altered basic glass.
Foraminiferous Volcanic Mud-rocks These deposits, which represent the “volcanic muds” forming around the coasts of volcanic islands, are more or less consolidated clay-rocks. They contain in varying numbers the tests of foraminifera with occasionally pteropod-shells. The former are usually minute and of the “Globigerina” type; but in some rocks larger bottom-forms prevail. ”

“ In the higher part their grain is ·5 to 1 mm., and they are more basic in character and come near to the palagonite-tuffs. At an elevation of 200 feet they form inland cliffs, 50 feet high, in which are imbedded blocks, 2 feet across, of a blackish pyroxene-andesite with a specific gravity of 2·73, and belonging to the prismatic sub-order of the orthophyric order of the hypersthene-augite andesites. It is remarkable for the pyroxene prisms of the groundmass, and shows a little interstitial glass. These cliffs are well displayed behind Navelatha, about half a mile from Tathelevu. ”

“ The larger glass fragments, which vary in different rocks from 1 or 2 to 4 or 5 mm. in size, have been crushed in situ, the broken portions often remaining more or less in position. These fragments are invested by palagonite and have eroded borders, as shown in the figure on page 342. The glass is bottle-green, non-vacuolar, fuses readily, and only at times displays incipient crystallisation. The explanation of the origin of these rocks is attempted in Chapter XXIV. They contain neither carbonate of lime nor organic remains. ”

“ The venerable Sir Henry Holland recorded in 1810, that the Krísuvík formation occupied high ground ‘composed principally of the conglomerate or volcanic tufa which has before been noticed:’ this palpable reference to Palagonite has not been worked out as it deserved to be. The ‘vivid word-pictures’ of older travellers are either written in the fine style of former days, or the subjects of description have lost youth and vigour. The ‘tremendous proofs of what is going on beneath us’ are now, or have become, phenomena on a very mild scale ”

“ The long stout felspar-lathes, which are on the average ·3 to ·45 mm. in length, give lamellar extinctions of 15° to 20° (medium andesine) .... The copious smoky glass is rendered partially opaque by the abundant development of rods and skeletal crystals of magnetite, and shows the fibrous devitrification arising from the formation of incipient microliths. In some rocks there appear in the smoky glass brownish-yellow patches of the residual magma which under the microscope cannot be distinguished from palagonite. ”

“ It is, however, not so level-topped as it appears to be; but the gradual variations in elevation between 2,000 and 3,000 feet, when spread over a length of some miles, are more or less lost in the general outline of the range as viewed from the coast. Basic agglomerates are principally exposed on the lower slopes; whilst higher up, reaching often to the summit of the table-land, occur palagonite-tuffs containing tests of foraminifera and molluscan shells, massive basic rocks being exposed in places. ”

“ Regarded as components of the pitchstone-tuffs and palagonite-tuffs these rocks have a very extensive distribution in the island; but in the massive state they are hardly ever to be found, whilst in the form of agglomerates they are only frequent in certain localities, as in the cliffs of the Korotini Bluff, in the vicinity of Mbale-mbale, on the slopes of Soloa Levu, and in the dividing ridge between the Mbua and Lekutu plains. On rare occasions they are to be found in a rubbly condition, as in the upper part of a basaltic flow described on p. ”

“ Inland, towards Narengali and Va-lili, these rocks are often more or less glassy and take the form of pitchstones; whilst the agglomerates have the same character. The first probably represent submarine flows of basaltic lava which have spread far and wide over the Ndreketi plains. The inland rocks are, as is pointed out below, the products of vents that, as in the case no doubt of Mount Nakambuta, rose out of a shallow sea. The palagonite-tuffs and clays, often foraminiferous, which cover the Ndreketi plains, are extensively represented in the lower levels up to 400 feet or more. ”

“ Such rocks evidently represent ancient flows. They give place as one proceeds east to porphyritic semi-ophitic doleritic rocks of the same genus and to semi-vitreous basic rocks. About half a mile west of Nukunase a vesicular doleritic basaltic andesite forms a spur protruding at the coast. It is semi-ophitic and contains in the smoky glass of the groundmass little irregular cavities filled with a yellowish residual magma like palagonite in character. (It is referable to genus 12 of the augite-andesites, described on page 275.) ”

“ The outer wall, raised 150 to 200 feet, is one mass of soft sulphur covered by black sand; every footstep gives vent to a curl of smoke, and we do not attempt to count the hissing fumaroles, which are of every size from the thickness of a knitting-needle upwards. With the least pressure a walking-stick sinks two feet. We pick up fragments of gypsum; alum, fibrous and efflorescent; and crystals of lime, white and red, all the produce of the Palagonite, which still forms the inner crust ”

“ The normal rock consists now chiefly of short broad rather irregular crystals of partly altered plagioclase, with numerous microliths and aggregated small crystals of pseudomorphous actinolite. Here and there, however, portions of plagioclase crystals still remain but little altered, as well as crystals of diallage in which the change to hornblende has not been completed. Every stage of the change from plagioclase to the saussuritic mineral can be traced in various parts of the slide. There are occasional microliths and larger grains of magnetite ”

“ There are only four minerals which deserved to be ranked as essential and characteristic constituents: they are plagioclase, feldspar, pyroxene, generally in the form of hypersthene hornblende, and olivine. The kind of lava which has by far the widest distribution upon the slopes of Mt. Shasta is composed essentially of plagioclase, feldspar and hypersthene, with some angite, and belongs to the variety of volcanic rocks which, on account of composition, and the place where first discovered, has been designated hypersthene andesite. ”

“ When leucite or nepheline replaces plagioclase, the rock becomes a leucite-basalt, [1] or nepheline-basalt. Some basalts have a glass paste, or "ground-mass," in which the minerals are enclosed.
The lava of Vesuvius may be regarded as a variety of basalt in which leucite replaces plagioclase, although this latter mineral [Pg 261] is also present. Zirkel calls it "Sanidin-leucitgestein," as both the macroscopic and microscopic structure reveal the presence of leucite, sanidine, plagioclase, nephiline, augite, mica, olivine, apatite, and magnetite. ”

“ Thin veins of serpentine often show in the latter; and films of the same not seldom coat joint-faces. The dyke seen in the cliff may be taken as a type of the former. On microscopic examination it proves to be a felspar basalt,—the plagioclase being, as a rule, clear, though with polarized light dull in colour, the augite little changed, and dull green spots replacing probably the olivine. There are also grains of magnetite, with brown stains from one of the other ferric oxides; and cracks are overspread by an infiltrated serpentinous mineral. ”

“ Kaolinization is the decomposition of the felspars, which are the commonest minerals of igneous rocks, into kaolin (along with quartz, muscovite, &c.) ; it is best shown by granites and syenites. Serpentinization is the alteration of olivine to serpentine (with magnetite) ; it is typical of peridotites, but occurs in most of the basic rocks. In uralitization secondary hornblende replaces augite; this occurs very generally in diabases; chloritization is the alteration of augite (biotite or hornblende) to chlorite, and is seen in many diabases, diorites and greenstones. ”