Page:EB1911 - Volume 22.djvu/119

From Wikisource
Jump to navigation Jump to search
This page needs to be proofread.

finely crystalline aggregates of white mica and other secondary products as in the well-known liebenerite-porphyry of Tirol and gieseckite-porphyry of Greenland. The felspars of these rocks are albite, orthoclase and anorthoclase. and they often contain soda-augite and amphiboles. Elaeolite-porphyries occur along with nepheline-syenites in such districts as the Serra de Monchique, south Norway, Kola, Montreal. Allied to them are the tinguaites (so called from the Serra de Tingua, Rio de Janeiro, Brazil), which are pale green rocks with abundant alkali felspar nepheline, needles of green aegirine, and sometimes biotite and cancrinite. As a rule, however, these are not porphyritic. Some authors group the tinguaites with the aplites rather than the porphyries. Grorudites are quartz-tinguaites free from nepheline, and solvsbergites are tinguaitic rocks in which neither quartz nor nepheline occur. The two last varieties have been described from the Christiania district in Norway, but tinguaites are known with nepheline-syenites in many parts of the world, e.g. Norway, Brazil, Portugal, Canada, Sweden, Greenland.

The following analyses of porphyries of different t es will show YP

the chemical composition of a few selected examples:- SiO2 A1203 Fe2O3 FeO MgO CaO KZO Na2O H20 I. 72-51 I3'3I tr. 3-87 1-50 0-60 6-65 0-43 0-60 II. 67-18 16-65 o-55 2-15 1-54 2-35 2-91 4-03 0-75 Ill. 71-60 13-60 2-40 - 0-21 2-30 3-53 5-55 0-70 IV. 58-82 21-06 3-26 o-70 1-38 3-03 3-70 6-83 1-26 L-, iJ

V. 45-18 23-31 6-11 1-45 4-62 11-16 5-94 1-14 VI. 54-46 19-96 2-34 3-33 0-61 2-12 8-68 2-76 5-20 VII. 75-20 12-65 1-53 0-28 O'26 0-60 4-14 5-67 0-12 I., Elvan or granite porphyry (with pinite after cordierite)-Prah sands, Cornwall. ll., Granophyrv-Armboth, Cumberland. III., Granophyre-Carrock Fell, Cumberland. IV., Rhomben-porphyry -Tonsterg, Norway. V., Elaeolite porphyry-Beemerville, New Jersey. VI., Tinguaite-Kola. VII., Grorudite-Assynt, Scotland. Porphyriles.-The porphyrites as above mentioned are intrusive or hypabyssal rocks of porphyritic texture, with phenocrysts of plagioclase felspar and hornblende, biotite or augite (sometimes also quartz) in a fine ground-mass. The name has not always been used in this sense, but formerly signified rather decomposed andesitic and basaltic lavas of Carboniferous age and older. Both the red porphyry and the green porphyry of the ancients are more properly classified in this group than with the granite-porphyries, as their dominant felspar is plagioclase and they contain little or- no primary quartz. Porphyrites occur as dikes which accompany masses of diorite, and are often called diorite-porphyrites; they differ from diorites in few respects except their porphyritic structure. The phenocrysts are plagioclase, often much zoned with central kernels of bytownite or labradorite and margins of oligoclase or even orthoclase. In a special group there are corroded blebs or porphyritic quartz: these rocks are called quartz-porphyrites, and are distinguished from the granite-porphyries by the scarcity or absence of orthoclase. The hornblende of the porphyrites is often green but sometimes brown, resembling that of the lamprophyres, a group from which the porphyrites are separated by their containing phenocrysts of felspar, which do not occur in normal lamprophyres. Augite, when present, is nearly always pale green; it is not so abundant as hornblende. Dark brown biotite is very common in large hexagonal plates. Muscovite and olivine are not represented in these rocks. The ground-mass is usually a crystalline aggregate of granular felspar in which plagioclase dominates, though orthoclase is rarely absent. The Alpine dike rocks known as ortlerites and suldenites are porphyrites containing much green or brown hornblende and augite; these, however, hardly require a distinctive designation. Diorite-porphyrites have almost as wide a distribution as granite-porphyries, and occur in all parts of the world where intrusions of granite and diorite have been injected; they are in fact among the commonest hypabyssal rocks.

To gabbros and norites certain types of porphyrite correspond which have the same mineral and chemical composition as the parent rocks but with a porphyritic instead of granitic structure. Gabbro-porphyrites are not numerous; or rather most of these rocks are described as porphyritic basalts and dolcrites. The beerbachites are 'finely granular dike rocks resembling gabbros in all respects except in their being less coarsely crystalline. Norite-porphyrites have porphyritic plagioclase (labradorite usually) with hypersthene or bronzite, often altered to bastite. They accompany norite masses in Nahe (Prussia) and Tirol. They have vitreous forms which are described as andesiticpitchstones or hypersthene-andesites.

s10, |.-uio. Fe2o.|Feo "CaO Mgo KZO Nazi) H20

I- 64-94|17-50 0-69 3-94 2'S9 2-83 3-II 3-44 I-36 II. 61-58 18-84 4-68 — 6-59 2-04 1-49 4-27 1-61 LIII. 56-85 16-70 5-9: 7-13 5-97 3-25 1-91 2-78 o-54 I, Quartz-porphyrite-Lippenhof, Schwarzwald. II., Porphyrite-Esterel, France. Ill., Norite-porphyrite-Klausen, Tirol. S F) (J.

PORPOISE (sometimes spelled Porpus and Porpesse), a name derived from the O. Fr. porpeis, for poropeis, i.e. pig-nsh, Lat. porous, pig, and piscis, fish; the mod. Fr. marsouin is borrowed from the Ger. meerschwein, although the word is commonly used by sailors to designate all the smaller cetaceans, especially those numerous species which naturalists call “ dolphins, ” it is properly restricted to the common porpoise of the British seas (Phacaena communis, or P. phocaena). The porpoise, when full grown, attains a length of 5 ft. or more; the dimensions of an adult female specimen from the

F IG. 1.-The Common Porpoise (Phocaena communis). English Channel being: length from nose to notch between the flukes of the tail, 62% in.; from the nose to the front edge of the dorsal fin, QQ in.; height of dorsal fin, 4% in.; length of base of dorsal fin, 8 in.; length of pectoral fin, gi in.; breadth of pectoral fin, 3% in.; breadth of tail flukes, I3 in. The head is rounded in front, and differs from that of dolphins in not having the snout produced into a distinct “ beak ” separated from the forehead by a groove. The under jaw projects about half an inch beyond the upper. The mouth is wide, bounded by stiff immobile lips, and curves slightly upwards at the hinder end. The eye is small, and the external ear represented by a minute aperture, scarcely larger than would be made by a pin, about 2 in. behind the eye. The dorsal fm, near the middle of the back, is low and triangular. The llippers are of moderate size, and slightly sickle-shaped. The upper-parts are dark grey or nearly black according to the light in which they are viewed and the state of moisture or otherwise of the skin; the under-parts pure white. The line of demarcation between these colours is not distinct, washes or splashes of grey encroaching upon the white on the sides, and varies somewhat in different individuals. Usually it passes from the throat (the anterior part of which, with the whole of the under jaw, is dark) above the origin of the flipper, along the middle of the flank, and descends again to the middle line before reaching the tail. Both sides of the flippers and flukes are black. The anterior edge of the dorsal fm is furnished with a row of small rounded horny spines or, rather, tubercles, of variable number. One of