Porphyritic Basalt Research Articles

GEOLOGY OF THE REYDARFJÖRDUR AREA, EASTERN ICELAND

A succession of nearly 15,000 feet of volcanic rocks, mostly plateau-basalt lavas, is described from the Reydarfjördur area. This area is in the middle of the large Tertiary outcrop of eastern Iceland, and the rocks described constitute the lower parts of the exposed succession there. They dip uniformly west-south-west at 3–7 degrees. Tholeiites, olivine-basalts, and porphyritic basalts rich in phenocrysts of basic plagioclase make up the bulk of the lava pile, and there is little sign of any systematic distribution of these types. Lavas of a particular type tend to form groups of flows, and these can be readily mapped in the field and clearly have a wide horizontal extension. Two long periods of acid volcanism are distinguished, both having given rise to a considerable bulk of acid and intermediate lavas and pyroclastic rocks. Two more acid volcanic episodes are represented in the mapped ground by acid tuffs, and a fifth is represented by the copious outpourings of acid and intermediate lavas and pyroclasts of the Thingmuli central volcano which was later built on the platform of plateau lavas described in this paper. The bulk of the lavas are regarded as the product of fissure eruption, and two examples have been found of a lava connected to its dyke feeder. The dyke swarm in the area is estimated to contain over 500 dykes, mostly basic, with a general northerly trend.

A structural investigation of the porphyritic basalts of Arthur’s Seat, Edinburgh

I. Introduction. It is well known that flow can impart a regular arrangement to solid bodies immersed in a moving fluid. This regularity can be either planar or linear, or a combination of the two (R. Balk, 1937). Early crystals (phenocrysts) in a porphyritic basalt may have been oriented by the flowing magma; as the latter froze, so the flow pattern would be preserved, and can now be measured. Structure maps and profiles, based on such measurements, give details of the formation of volcanic bodies that are not otherwise obtainable. With few exceptions, the flow structures investigated on Arthur’s Seat are planar. The intersection of a flow plane with the plane of an exposure gives a linear pattern on the latter. The orientation of the flow plane can be determined if this linear pattern can be found on two neighbouring, non-parallel exposure surfaces. The accuracy is greatest when the two exposures planes are at right angles both to each other and to the flow plane. The margin of error of the measurements actually made did not exceed 15°. Where flow structures are as feebly developed as in the Arthur’s Seat basalts, a higher degree of accuracy is not possible. However, if a sufficient number of measurements be made, this degree of exactitude is sufficient. Though in many instances it may seem at first glance that no structure pattern is visible on a given surface, careful observation often reveals it. The technique of observing and measuring cannot adequately be described in

Volcanics of the Buck Hill Quadrangle, Brewster County, Texas: ABSTRACT

The volcanic rocks in the north and west parts of the Buck Hill Quadrangle are a southeastern extension of the Davis Mountains of trans-Pecos Texas. Field evidence suggests a division of the volcanics of this area into two regions: (1) west of Calamity Creek the volcanic series, consisting of flows of trachyte prophyry, andesite porphyry, basalt (five flows), and rhyolite porphyry separated by beds of ash, tuff, and breccia, has a combined thickness of 2,100 feet; (2) east of Calamity Creek the rock series is distinctly different: here porphyritic olivine-free diabasic basalt flows (?) and intervening beds of ash and fresh-water conglomerates predominate and are 552 feet thick. A major stratigraphic break separates the volcanic rocks from the underlying Upper Cretaceous rocks, and everywhere a basal conglomerate marks this unconformity. No difference in the dips of the underlying Cretaceous rocks and the volcanics is noticeable in the field, therefore the break at the base of the volcanics is considered one of disconformity rather than of angular unconformity. The age of these volcanic rocks is not known, but from available data it seems probable that they are of Cenozoic age. End_of_Article - Last_Page 1712------------

Note on corroded ferromagnesian phenocrysts in a Scottish basalt

I. Introduction. The writer has recently made a study of a large number of slices of the Lower Carboniferous porphyritic basalt lavas of North Ayrshire and adjoining areas (MacGregor, 1930,1 pp. 89-99). In these rocks phenocrysts of augite, olivine and plagioclase felspar may often be seen to have sub-rounded outlines, or to have embayments and enclosures of groundmass. One rock-slice in particular, because it contains an exceptionally good example of a honeycombed augite phenocryst, deserves a more detailed description than could be accorded to it in the North Ayrshire Memoir (MacGregor, 1930, p. 97). The rock from which the slice (20,196)2 was cut was collected by Mr J. E. Richey near Craig of Neilston, Renfrewshire, in ground surveyed by him for the Geological Survey. It is an olivine-basalt of Dunsapie type (MacGregor, 1930, p. 91), and contains large phenocrysts of olivine, augite and felspar. II. Description of the Rock. The chief feature of the slice is a large phenocryst of pale fawn-brown augite about 1 cm. across (Plate XLVII., Fig. 1). The phenocryst is traversed by ramifying passages that locally pinch out almost entirely. The material in these cavities has crystallised mainly as felspar, granules of augite, and iron-ore, and closely resembles the groundmass. This material enclosed in the augite is in fact seen to be in communication with the groundmass at the right-hand side of the photograph (Plate, XLVII., Fig. 1). There is a decided tendency for little felspar laths (traversed by numerous delicate apatite needles, and accompanied by

XIII. The Igneous Geology of the Cumbrae Islands, Firth of Clyde

Introduction. The picturesque and conspicuous Cumbrae Islands, in the Firth of Clyde, have often been mentioned in geological literature, and many of their geological features and a few of their igneous rocks described; but there has never been a comprehensive statement of the characters and relations of the igneous suite as a whole in connection with the great Clyde Carboniferous lava-plateau of which it forms a part. Sir A. Geikie (1897, pp. 368-9, 385, 407-8) makes repeated references to the igneous phenomena displayed in these islands, and gives an excellent sketch of the Little Cumbrae, showing its fine regular lava escarpments. In regard to the characters and ages of the several groups of dykes (and sills) in the Great Cumbrae he follows the work of W. Gunn (1903), which was published later in a Survey memoir. Gunn distinguished four groups of dykes—(1) Trachyte (and bostonite), having a general E.N.E. direction; (2) porphyritic basalt, trending in the same direction; (3) E.W. dolerite dykes; and (4) N.N.W. dykes, probably of Tertiary age (Gunn, 1903, pp. 61-2). These are arranged in order of age, but we shall have reason later to question the general priority of the trachytes and bostonites. A few petrographical details are given in an appendix to the memoir (pp. 177-8). In these Transactions Mr. James Neilson has contributed a short paper on the Little Cumbrae and its minerals (1893), and Mr. John Smith has described the remarkable flow structure in a felsite dyke north of Downcraig Ferry in the This 250-word extract was created in the absence of an abstract

On a series of volcanic rocks from the neighbourhood of the Lucalla River, Angola

Last year, while working through the geographical collection of rocks in the Geological Department of the Imperial College, I found an interesting series of volcanic rocks from the neighbourhood of the Lucalla River in the district of Cambambe, Angola, which hitherto has not been described. The specimens were collected about the year 1860 by J. J. Monteiro, a former student of the Royal School of Mines, who devoted many years to the exploration of Angola. The collection, though consisting of only ten specimens z comprises porphyritic basalts, alkalitrachytes, a nephelinite allied to monchiquite, and an augite-andesite.

III.—On the Microscopic Structure of the Lambay Porphyey (or Porphyrite)

This very handsome rock forms almost the whole surface of the Island of Lambay and of the adjoining coast, a few miles to the north of Dublin Bay. It is also laid open in the railway cutting at Donabate Station; and has received considerable notice owing to its intrinsic beauty as an ornamental stone, and its proximity to the Irish metropolis. I am informed by Kev. Dr. Haughton that he has identified specimens from the tessellated pavement of St. Kevin's Church, Glendalough, of the seventh century, which were submitted to his in spection by Sir W. Wilde. A polished table, together with a large block presenting two sides cut and polished, are placed in the gallery of the Royal College of Science in St. Stephen's Green; where, to those familiar with the marbles of Italy and Greece, the appearance of these specimens cannot fail to recall the green porphyry (Marmor Lacedæmonium viride) so largely used in ancient decorative works of art, and often reset in modern ones, the source of which is in the neighbourhood of Sparta and Marathon. The two rocks, however, though resembling each other in appearance, are essentially distinct as regards mineral composition, M. Delesse having proved that the Grecian porphyry contains augite as an essential, and is apparently a porphyritic basalt, while the Irish porphyry is altogether destitute of this mineral. As regards geological age, the Lambay porphyry appears to have been intruded amongst the Lower Silurian rocks at a period antecedent to that of the Old Bed Sandstone, as this latter formation overlies the porphyry without appearing to have undergone any alteration along the surface of contact.