Augite Crystals Research Articles

Acid Clay: An Agent in Chemical Weathering

A generalized consideration of the nature, importance, and distribution of the acid clays was made. The physical-chemical nature of the acid clay was compared with some of the more familiar ordinary acids. A study of the effect of the acid clay in chemical weathering by mixing primary minerals with suspensions of acid clays revealed that the acid clay was extremely effective as an agent in weathering. In a comparatively short time interval of 107 days 3.4 per cent of the crystal-lattice calcium of anorthite was transferred from the mineral crystals to the adsorbed atmosphere of the clay. The hydrogen adsorbed on the clay was effective in bringing about removal of calcium from crystals of hornblende and augite. Reactions with biotite and microcline were found to be extremely slow. The importance of the acid clay and its place in natural weathering, giving consideration to equilibrium conditions, completeness of reactions, etc., are discussed.

Leucite-Basanite in East Lothian

ONE mile due east of Tantallon Castle, on the East Lothian coast, there stands a conspicuous tower or beacon. Its purpose is to afford an offshore warning to mariners. It has been constructed upon the seaward extremity of a northerly-extending ridge of volcanic ash or agglomerate. In recent years the rocks exposed here have been carefully examined by a well-known Edinburgh geologist, the late Mr. T. Cuthbert Day, and his results are reported in an interesting paper entitled “Volcanic Vents on the Coast, from Tantallon Castle eastwards to Peffer Sands, and at Whitberry Point” (Trans. Edin. Geol. Soc., xii, 1930, p. 213). Mr. Day thought that the ash and agglomerate lying south of the beacon represented the materials belonging to two contiguous but distinct volcanic vents of Lower Carboniferous age. To the twin orifices he applied the designation—“ The Car Double Vent.” In the northerly vent there occur two basaltic intrusions intersecting the pyroclastic rock. One of these, an irregular 4 ft. dyke, trends north-west hard by the easterly side of the beacon. The other, an oval plug covering 500 square feet, or thereby, is situated about 100 yards to the south-east. Both masses were determined petrographically by Mr. Day as “ dark red basalt with conspicuous dark crystals of augite, somewhat decomposed, but appearing under the microscope to approach a limburgitic type ”. It is in reference to these “ limburgitic ” intrusions that I present the following brief note, because, lately, I have discovered that they consist of leucite-basanite, a rock of basic felspathoidal character not previously known to occur in this country.

The crystallization of basalts

This classic paper presents Fenner9s views of magmatic differentiation against those of N.L. Bowen (1928). Fenner presents the original case for iron-enrichment trends in residual basaltic liquids, contrasted with an iron depletion trend leading to rhyolite. He discusses the general crystallization process of basaltic lavas, mentions Clarence King9s (1878) identification of olivine settling in Hawaii, and considers the results of the crystallization of augite and olivine from a picrite-basalt of East Maui.

The Petrography of the Austral or Tubuai Islands (Sourthern Pacific)

I. Introduction. The following account of the petrography of the islands of Rapa and Rurutu in the Austral Group is based on the collections made by one of us in April and May 1925 during the St. George Expedition to the Southern Pacific. A general account of the geology of these islands has already been given by one of us (Chubb, 1927), and references to previous literature will be found in that paper. In Rapa the district most thoroughly worked was that between Mount Vairu and Mount Taga and the head of the harbour, though collections were made from many other parts of the island. In Rurutu only the area between Moerai Bay and the districts of u and Avera was explored. The few occurrences of sedimentary rocks have been described in the paper already mentioned. A brief account will be given of the pyroclastic rocks which were found at a few localities, after which the small intrusions and the volcanic rocks, forming the main bulk of the island, will be described in detail. The volcanic rocks are often deeply weathered to a red clay, especially in the higher parts of Rurutu, where no expesures of fresh rock were found above 600 feet. Elsewhere, however, exposures of unweathered rocks are fairly common, and the rocks selected for description are remarkably fresh and very suitable for chemical and petrographical study. II. Rapa Pyroclastic Rocks. A brown-drab basalt-breccia [840] containing numerous crystals of augite (5 mm. long) and small olivines, in a

Augite from Bail Hill, Dumfriesshire

The crystals to which this paper refers are found at Bail Hill, about three miles north of Sanquhar, in Dumfriesshire. At this locality volcanic rocks of Arenlg age outcrop in the midst of the greywacke of the Southern Uplands. The rocks are mainly lavas, and include some coarse-grained tufts which are composed chiefly of augite- and hornblendeandesites. Ill the andesitic fragments, there arc numerous porphyritic crystals of a black augite, which call be easily separated from the rest of the rock. Isolated crystals of augite and hornblende occur also in the matrix of the tufts. No mention of this occurrence is made in Heddle's 'Mineralogy of Scotland', and apparently the only reference to it in previous literature is the short petrographic description in the Geological Survey Memoir.

Optically uniaxial Augite from Mull

The mineral hero described was recently collected by the officers of the Geological Survey during their work for the six-inch map of the Island of Mull. It is present as minute phenocrysts in a dark, glassy, magnetite-bearing rock/which is intruded as sheets in the Tertiary lavas nearly one mile south-south-west of Pennygael.The small rounded crystals of augite are seldom more than ½ mm. in diameter and are traversed by irregular cracks. They contain a few negative crystals, and are occasionally darkened, probably with included magnetite. They appear to have been reabsorbed to some extent by the matrix.

VI.—Geology and Petrology of the Intrusions of the Kilsyth-Croy District, Dumbartonshire

The general appearance of this rock in thin section is of an interlocking framework of plagioclase, augite, and iron-ores, with micropegmatite occupying the interstices. The mutual relations of the felspar and augite vary considerably. The felspar may be entirely enclosed in the augite. In this case it is very basic and only faintly zoned. The most common relation is for the augite to enclose the terminations of the felspar laths, the sub-ophitic structure of Watts. But frequently idiomorphic crystals of augite are found partly or wholly enclosed in felspar. The latter is then more acid than the ophitic felspar, and is broadly zoned. These varying relations may frequently be found within the limits of a single slide.

The occurrence of isolated augite crystals at the top of the Permo-Carboniferous upper marine mudstones at Gerringong, New South Wales

The occurrence of isolated augite crystals at the top of the Permo-Carboniferous upper marine mudstones at Gerringong, New South Wales

Observations on the Tiree Marble, with Notes on others from Iona

I. Introduction. Although so well known, and represented in many mineralogical collections, the Tiree Marble has never been described in detail. Macculloch says that the Tiree flesh-coloured marble ‘resembles the greater number of the primary limestones found in gneiss and mica-slate, and may be considered as a large nodule ... The nodule of limestone ... appears to be an irregular mass of about 100 feet in diameter, and is surrounded on all sides by gneiss.’ [The marble] ‘occasionally contains imbedded lumps of granite or gneiss, similar to those which occur in the limestone of Glen Tilt. These are always visible at the surface, from their superior power of resisting the action of the atmosphere.’ He also describes the exposures of limestone in a field south-east of Balephetrish Hill. He says that ‘it is possible that the masses of limestone thus found in gneiss have once been stratified, and that they have suffered some posterior changes by which the appearances of this disposition have been obliterated.’ Prof. Bonney has given a valuable description of the dynamic phenomena so well illustrated in thin sections of the Tiree Marble. Prof. Cole & Prof. Sollas have suggested that the Tiree Marble may originally have been a wind-blown coral-sand rock, with abundant rounded crystals of detrital augite, etc., such as was found on the east side of Mer, one of the Murray Islands. This theory appears untenable; for it is impossible to imagine a volcanic origin for the majority of the accessory minerals, which, with

Notes on the Rotuman Rocks collected by Mr. Stanley Gardiner

1. Olivine-Dolerite from Kugoi. This is a light-greyish, moderately coarse-grained dolerite, containing much felspar. The rock is quite fresh, and consists of plagioclase-felspar, augite, olivine, magnetite, and apatite. Two generations of felspar are very distinct—the crystals belonging to lhe earlier showing good contours. It is probable that this is an intrusive rock. 2. Basalts from Tarasua Point, Mafiri Cave, and An Hufhuf. These are very vesicular basalts, black or grey in colour. The specimen from Tarasua Point was found by Dr. W. Pollard to contain 48·86 per cent. of silica; the rock is dark in colour and consists of minute, but long, plagioclase-felspars (with a few larger crystals of the same mineral), augite, and magnetite. The small felspars have a parallel arrangement. The specimen from Mafiri is a fine-grained rock, in which the felspars have no regular arrangement. 3. Olivine-Basalts from Savelei Point, base of Hoi, and base of Yavasse (opposite Sol Kopi). The specimen from Hoi is a greyish compact rock with light green crystals of olivine. Microscopic examination shows that it is fine-grained and not decomposed; it consists mainly of a mass of small, lath-shaped, plagioclase-felspars and augite, with larger crystals of augite and olivine. Magnetite is abundant. The felspars have a fairly well-marked parallel arrangement. The rock from Savelei Point is darker and somewhat vesicular, resembling in general appearance the specimen from Tarasua Point, but containing olivine. 4. Ashes from Sol Kopi, Afaga, Howa, Kugoi, Kiliga Point, and Sororoa. These are light-coloured rocks, varying in texture and

The Laccolites of Cutch and their Relations to the other Igneous Masses of the District

abstract The Author has observed thirty-two domes of various kinds in Cutch, distributed as follows:—(i) those connected with the northern islands; (ii) those of Wagir; and (iii) those along the northern edge of the mainland. They are divisible into four classes: ( a ) those which are so elongated on the line joining adjacent ones that they seem to be mere modifications of anticlinals, though the supposed anticline is not really continuous; ( b ) those which lie in a line, but are not elongated in that direction, and often in no other; ( c ) those which are related to a fault, which cuts them in half; and ( d ) those which are not in any particular relation to each other, or to any other stratigraphical feature. The domes vary in degree of perfection: some are irregular, while some have the strata running in concentric circles, the outer and newer strata dipping away from the inner and older. In no less than ten of the thirty-two domes igneous bosses are found occupying the centre, and these are distributed amongst all of the above classes. The Author gives reasons for maintaining that the domes are the results of intrusion of igneous rocks in the form of laccolites, and are not anticlinal folds which have afterwards been affected by cross-folds. The domes are contrasted with igneous peaks which occur in abundance in a different part of the area, usually at a higher horizon of the strata and at a higher level above sea. These are probably volcanic pipes through which the lava was forced and extruded at the surface. The Author compares the rocks of the bosses with those of the dykes and flows. Both are principally perfectly fresh dolerites, but the former are distinguished by the presence of intergrowths of micropegmatite as the last stage of consolidation, as in the ‘Konga diabases.’ There is also among them a felsite-breccia with micropegmatite developed in the cracks. He considers that nearly all the igneous rocks of Cutch have been derived from a single magma, which in a solid condition must have contained large crystals of augite, olivine, and ilmenite in a groundmass of lime-felspars, and have been throughout of a basic character. Such a magma originated in more than one centre. One was possibly not far from the Sindree basin, whence lines of weakness diverged. Along these, owing to the thickness of the strata, there was no extrusion at the surface, and laccolite-domes were formed. Where the rock reached higher levels, it spread out into sheets between the domes and aided in the production of synclinals. Another centre was west of Bhuj, where the rock reached the surface without materially disturbing the sedimentary rocks, and formed the so-called ‘stratified traps.’ There were also areas here, especially to the south, where escape was impossible and domes were formed. The Author observes that his conclusions, if correct, may be applied to explain the source of the Deccan trap without eruptive centres. It may have been forced out from innumerable orifices as from a sieve, none of these being so much larger than others as to make a definite centre.

II. On the Occurrence of Pyroxene or Augite, in Large Crystals, in Two Localities in Western Scotland

The occurrence of the mineral Pyroxene or Augite, in large crystals, amongst the basic igneous rocks of this district of Western Scotland, seems to be far from common. I formerly only knew of one intrusive boss of dolerite, near Sculliongour on the Campsie Fells, which contained them. In it many of the crystals reach some half-inch in diameter, and specimens of these are now exhibited. This rock I brought under the notice of Mr. S. Allport, F.G.S., Sir Josiah Mason Science College, Birmingham, whilst he was preparing his paper on the microscopic structure and composition of the “British Carboniferous Dolerites” ( Quarterly Journal of the Geological Society , 1874, vol. xxx., p. 529). Mr. Allport, in referring to my specimens, remarks: “A very beautiful porphyritic dolerite occurs on the south slope of the Campsie Hills, near Sculliongour. It is coarsely crystalline in texture, with large disseminated crystals of augite. The plagioclase (felspar) is well crystallised, very slightly altered, and exhibits brightly-coloured striæ in polarised light. Olivine, is abundant and unusually well-preserved, some crystals being almost unaltered, others rather more so, while a few exhibit more advanced stages of its conversion into a serpentinous pseudomorph. The augite is well crystallised, and is remarkable for the number of cavities, grains of magnetite, and small felspar crystals it contains.” Mr. Allport then concludes his description by stating that “the relation of this rock to the surrounding masses could not be traced, and its unusually fresh condition might lead one to suspect for it a more This 250-word extract was created in the absence of an abstract

The Origin of certain Marbles1

AMONGST the interesting collection of rocks brought home by Prof. Haddon from Torres Straits are some fragments of wind-blown coral-sand rock from Thursday Island. They have a deceptively oolitic appearance, and the majority of the grains being of a red colour give a prevailing warm tint to the stone, and thus render more conspicuous by contrast a number of dark green, worn, and rounded crystals of augite, which are scattered irregularly through it. The appearance of this handsome rock is sufficiently striking, but it gains greatly in interest from its suggestive resemblance to the famous Tiree marble, wherein likewise green grains of pyroxene are set in a flesh-coloured matrix of altered limestone. The comparison is confirmed and enhanced by an examination of thin slices; in the recent limestone the calcareous grains are found, as so commonly happens with these coral-sand rocks, to consist of rounded fragments of calcareous Algæ, and worn tests of various species of Foraminifera; mingled with these are more or less rounded crystals, not only of green augite, but also of olivine, felspar, and a finely crystalline glassy basalt; in the Tiree marble the green grains of pyroxene (salite) show beautifully rounded outlines, and are sharply separated from the surrounding matrix, into which they show no tendency to pass; crystals of felspar are also present—some fairly fresh, others, and these are the majority, corroded and almost entirely replaced by calcite, only the thin outer skin of the felspar preserving a fresh appearance; in some few cases, fragments of felspar partially penetrated by salite are met with. The calcareous matrix is finely granular, possibly dolomitic, but blotched and spotted by badly defined larger crystalline individuals of calcite, the outlines of which are sometimes obscurely rounded, so that although no trace of organic structure can now be recognized, yet on the whole the appearances are such as might be expected to be presented by a coral-sand rock, which had suffered metamorphic changes. Macculloch, in his detailed account of this rock, refers to its occurrence as an irregular mass, completely surrounded by gneiss; another white limestone occurs in the island, similarly disposed.

Contributions to the petrography of the Sandwich Islands

Dana describes specimens of lava, including lava stalactites, from Mauna Loa and from Kilauea, collected by J.D. Dana and E.S. Baker. The lavas of Mokuaweoweo and Kilauea are found to be strikingly similar. Samples from Mokuaweoweo include clinkstone-like basalt, chrysolitic basalt, lavas with minute feldspar and augite crystals in their cavities, and others. He describes elongated (skeletal) olivine crystals and dendritic growths of augite, illustrated by excellent drawings. Glass is found to contain both microlites and crystallites. The lava stalactites, also well-photographed, are thought to have been formed by a process of "aqueo-fusion" by highly heated water vapor. Dana also describes 1790 ejecta, including one block with apatite-bearing glass in vesicles, and lava from Haleakala on Maui and from Oahu.

Additional Note on Boulders of Hornblende Picrite near the Western Coast of Anglesey

In a short communication printed in this Journal (vol. xxxvii. p. 137) I described a large boulder of hornblende picrite which I found near Pen-y-Carnisiog in the autumn of 1880. Last summer I had an opportunity of spending an afternoon upon the western coast of Anglesey, and made use of it to examine the interesting section to the south of Porth Nobla. I did not, however, neglect to look out for boulders, as I thought it not impossible that I might meet with some more picrite. In this I succeeded beyond my expectations, and now lay the results before the Society, together with some remarks on the microscopic structure of the specimens collected. After quitting the railway at Ty Croes Station, I walked a short distance along the road to the south-west, and then turned up a field-way leading past a small farm called Bryn Gwyn. A short quarter of a mile from it a boulder (No. I.), perhaps roughly trimmed, has been utilized as the capstone of a gate-post. It measures 2 3/4 × 2 × 1 1/4 feet. A small fragment projected, which I was able to detach for examination without injury to the stone. The next (No. II.) was a well-rounded boulder lying on the sandy shore at Perth Nobla. This had a rather greener matrix than those which I had previously seen: and the porphyritic hornblende or augite crystals were not quite so large. It measured about 2 feet each way, and rose about 1 foot above the

II.—Note on the Pitchstones of Arran

In 1872 I published in the Geological Magazine (Vol. IX. p. 1), a brief account of the Arran Pitchstones, in which I figured and described the singularly beautiful groups of minute green crystals which are so characteristic of the rocks on the eastern shore of the island. I have since cut many additional thin slices, and have long been aware that my paper contains an error, which I will take the present opportunity to rectify. The groups of crystallites to which I refer are described on page 2 and illustrated, Plate I. Fig. 1, and contain the “long slender prisms of a green pyroxenic mineral” there mentioned. In the last paragraph, p. 9, these green prismsare again mentioned, and are said to be augite, instead of hornblende, as stated by Zirkel in his description of the same rock. The grounds for this conclusion were twofold, the absence of dichroism, and the presence of undoubted crystals of augite. After pointing out that the pale green crystals exhibit no trace of dichroism, I added, “Unless, therefore, it can be shown that some hornblende is not in the least dichroic. the mineral in question must, on this ground alone, be regarded as augite.” As regards the latter mineral, there can be no doubt whatever of its occurrence, for in addition to the crystal described in the paragraph just quoted, I may now add, that it is far from uncommon, perfectly characteristic examples having been observed by me in specimens from several different localities in the island.

III.—On Bowlingite, a new Scottish Mineral

In a recently exposed part of a quarry at Bowling, on the Clyde, my friend Mr. John Young, F.G.S., noticed a vein from an inch and half to half an inch in thickness, of a deep green mineral having the hardness and peculiar feeling of stealite, and having secured a good sample he forwarded it to me for analysis. It occurs filling a clean sided fisure on the north side of the quarry. The rock is described by Mr. Samuel Allport, F.G.S., as follows :-“At Bowling, on the North bank of the Clyde, three miles east of Dumbarton, the rock forming the “Hill of Dun,” is a coarse crystalline dolerite, varying in colour from brownish black to black, with distinct crystals of augite, olivine, and felspar diseminated through it. This porphyritic structure is well seen in thin sections. The base is rather finely crystalline and consists of small crystals or grains of the above mentioned constituents, together with numerous grains of magnetite. The large crystals of plagioclase are clear and beautifully striated, they frequently contain glass cavities, and here and there portions, of the base have been enclosed in them . . . . This rock affords excellent examples of serpentinous pseudomorphs after olivine in various stages of formation; the crystalline forms are unusually perfect, and the gradual encroachmeut of the serpentinous matter on the clear olivine may be very well seen. In one of the sections—microscopic—an old fracture has been filled with serpentine, thus forming a vein which traverses some of the large felspar crystals and the surrounding base. A few cavities are also filled with the same substance.”

IV.—On some Points in the Geology of the East Lothian Coast

The interesting relation between the Porphyrite of Whitberry Point, at the mouth of the Tyne near Dunbar, and the adjacent sedimentary rocks, was first noticed, we believe, by Professor Geikie, who speaks of it in the Memoirs of the Geological Survey of East Lothian, pages 40 and 41, and again in the new edition of Jukes's Geology, pp. 269. The volcanic mass which forms the point, consists of a dark felspathic base with numerous crystals of augite: it is circular in form, and is exposed for two-thirds of its circumference in a vertical precipice facing the sea, about twenty feet in height.

On the recent Eruption of Vesuvius in December 1861

Abstract On the 8th of December the ground in the neighbourhood of Torre del Greco was shaken by repeated earthquakes from dawn up to 3 o'clock in the afternoon. As many as twenty-one distinct shocks were counted there, but only one of them was felt at Naples. At the hour above mentioned the atmosphere over Torre was wrapt in complete darkness, clouds of ashes having been projected from several mouths which had opened on the slope of Vesuvius, a short distance above the town. Early on the next day (the 9th) I visited Torre del Greco, and leaving the town below, mounted towards the stream of lava which had in the preceding night poured forth from the apertures already mentioned. It had cooled so rapidly that I was enabled to walk upon the scoriaceous crust, though the interior was so hot that my stick took fire on being thrust into its cracks. After proceeding about 600 mètres to the N.E., I came to the smoking hills, which were still vomiting glowing scorim and ashes so abundantly as to prevent a near approach. The white steam and black ashes, ejected from them with violent shocks resembling the intermittent puffs of a steam-engine, rose in globular masses so as to form a columnar shaft, which, spreading laterally at a great height, reproduced the “pine” of Pliny. On my return to Torre del Greco I saw two new mouths open before me. About this time the central cone of Vesuvius, which had been tranquil hitherto, began to eject steam and ashes in thick clouds, attended by frequent flashes of lightning. The explosions of the new craters, as well as the flow of the lava, ceased almost wholly about the third day, viz. by the 12th of December. On the 16th heavy rains fell, the weather having been quite clear and tranquil up to that time. On a second visit made on the 23rd of December, I ascertained the number of opelfings, marked by minor cones with funnel-shaped craters, that had been formed on the flank of the mountain were about twelve--ranged close together on a line from E.N.E, to W.S.W. at the distance of about 600 metres S.S.E. from the old lateral crater whence the lava-stream of 1794 proceeded which had poured down on Torre del Greeo. Thus it appears that a fissure had on this occasion been formed in the side of the mountain, either on the prolongation of that of 1794, or parallel and close to it. The lavas produced by the two eruptions are also almost identical in mineral character, being very poor in leucite, but rich in augite crystals. On returning to Torre del Greco, I was surprised to find the principal fountains of the town overflowing with an excessive supply of water, as in general during eruptions the springs are rather apt to fail. Bubbles of carbonic acid gas were rising abundantly from the water. Many of the cracks which had been formed by the earthquakes in the pavement of the streets of the town were seen, it is said, to emit small flames (of carburetted hydrogen ?). It is certain that the shore beneath Torre del Greco was permanently elevated by above a metre'a long white line composed of mollusks and zoophytes attached to the rock, which only live under water, being now generally raised that much above the sea-level, through a space of at least two kilometres. The cone of Vesuvius continued to smoke at intervals for several days. On the 23rd of December ashes fell abundantly in the streets of Naples—a circumstance that has not occurred since 1822.

On the Geology of the Island of St. Mary's, one of the Azores.

The island of St. Mary's, the easternmost of the Azores, is about seven miles in length from east to west, and five in breadth from north to south, and is divided naturally into a plain and an elevated district, which, taken together, mark its aspect from the sea. The plain occupies about one-third part on the west side of the island, the other part being formed of an elevated ridge running from north-west to south-east, which rises to a double peak (Pico Alto) 1889 feet above the sea, and of which the sides decline on the north, east and south to mural cliffs of considerable height. The base appears to be a dark blue compact basalt, containing minute crystals of olivine. This basalt has in some parts assumed a contorted columnar structure and near the town of Villa do Porto a distinct prismatic character. Above this bed is a coarse red wacké, passing to a cellular brown amygdaloid, its softer parts thickly imbedded with splendent crystals of augite, and the cells of the harder mostly filled with arragonite in three varieties of crystallization, and carbonate of lime. In both are occasional imperfect crystals of hornblende. This would seem to be the second bed of the island, and to it succeed deposits of marine shells of variable thickness, in which the most conspicuous shells are the lesser and greater scallop, mixed in parts with an arenaceous cement and rounded nodules of basalt, breccia and amygdaloid, and interstratifled with films and layers