great part, of globuliferous or polyhedral elements. The globules are formed of fine laminæ, membraneous, with a greasy lustre, overlying like the coats of an onion, or presenting an undivided mass which resembles glass or porcelain; they are sometimes grouped confusedly, and, sometimes, by mutual pressure have taken a polyhedral form. In certain cases they are separated from one another in the center of a vitreous paste which resembles an enamel. This paste contains sometimes, crystals of Sanidine or Mica; sometimes Opals, grains of Obsidian (Marékanite), microscopic filaments analogous to those of Obsidian, and finally Spherolites. In the spherolitic Perlites, the Silica reaches 70 and 80%, and their density is much greater than that of Retinite, with which they are nevertheless classed, by reason of their common origin and the analogy of their general composition. TUFA, TRACHYTIC AND PUMICEOUS Fragments of pumiceous cinders or Trachytes, ejected from volcanoes, have been carried by the wind to considerable distances, and often again transported, modified, cemented by the water of the atmosphere or the earth. There have resulted from this action, masses of a clayey character, which are called Tufas, trachytic, phonolitic, pumiceous conglomerates. 34. Trachytic Tufa.-Friable or compact rock; grayish or ochre-yellow; earthy; having the appearance of volcanic mud solidified; enclosing, sometimes, crystals of Sanidine, Mica, etc. 35. Breccia, Trachytic Conglomerates.Angular fragments; rounded pebbles of Trachyte, mixed often with débris of different rocks, cemented by a paste which consists of Trachytic Tufa. 36. Phonolitic Tufa.- Débris of Phonolite mixed with fragments of crystals of Sanidine, Mica, Hornblende, and Magnetic Iron ore, united by an earthy paste which effervesces in acids. 37. Pumiceous Tufa.-Pebbles of Pumice, grayish, mixed with foreign substances (Mont Dore, France). with foreign substances (Mont Dore, France). 38. Pumice Conglomerates.-Fragments of Pumice agglutinated with débris of Trachyte or Obsidian, by a Pumice Tufa more or less apparent. 39. Volcanic Trass (Variety of pumiceous conglomerate). These are earthy rocks, dull grayish, yellowish, or brownish; more or less compact, formed of a pumiceous dust, decomposed, which encloses fragments of Pumice, Argillaceous schist, crystals of Feldspar, Mica, and carbonized wood. In the closed tube it yields water often containing ammonia, chlorine and sulphuric acid. It fuses with difficulty to a grayish enamel. It forms the bed of the river Rhine, where it is used as a hydraulic cement. According to Cordier, the paste of volcanic Trass seems to be trachytic rather than pumiceous, and the cement consists of hydrosilicate of Alumina, arising from the decomposition of the cinders, and sometimes of aluminite. Cordier recognizes rocks arising from the decomposition of Pumice or pumiceous cinders, calling the first Asclerines and the latter Alloites. CHAPTER II. PYROXENIC AND HYPERSTHENIC ROCKS. 40. Dolerite-A granular mixture, granitoid in texture, very brilliant on the fracture surface, having a somewhat spotted appearance. It is composed of Labradorite of a clear gray color in depressed crystals; Augite of a greenish-black color; and a small amount of Magnetite. Accessory elements: Calc-spar; Spathic Iron. Boiled in hydrochloric acid, this rock yields 40% of soluble ingredients; it effervesces for a short time. Accidental elements: Nepheline; Analcime in cubo-trapezohedrons, often limpid; Melanite in black rhombododecahedrons; Mica; Amphigene; Cubic Pyrites; Hornblende, etc. Varieties: Granitoide; Dol. Porphyritic; Dol. Mimosite; Dol. Anamesite, a fine-grained variety having a brilliant crystalline fracture, and passing to Basalt; Scoriaceous Dol. (Doloritic Lava). The lava of the island of Fogo, one of the Cape Verd islands, contains, according to M. Ch. Saint-Claire Deville, 54% of Labradorite; 19% Augite; 19% Olivine; 7% of titaniferous magnetic Iron. Its density is 3. That of Etna (eruption of 1865) has a density of 2.738 and contains 49.27% of Silica. Amygdaloidal Dolorites ;-Dol. with Analcime (Cyclophyre). Gray rock, porous and containing crystals of Anal cime. 41. Basalts.-Volcanic rocks, homogeneous, tenacious, of a bluish-black or gray color, generally dull, more or less porous, and more or less strongly magnetic; harder than steel. They are made up of grains, distinguished by aid of the microscope. The density is usually about 3; and the proportion of Silica is about 45%. Oxide of Iron from 6 to 22%. In the open tube it gives from 2 to 4% of water. They are easily fusible. In a celebrated memoir, Cordier first defined the mineralogical composition of Basalts. These masses consist of a mixture of grains, which are extremely fine, of Labrador Feldspar, Augite, and a certain relatively small quantity of magnetic Iron, more or less titaniferous. It very frequently encloses nodules of vitreous grains of Peridot of a yellowish-green or grayish color. Like the Melaphyres, the Basalts comport themselves as true Traps, and display themselves upon the surface in vast terraces one above the other; also they appear as great walls or masses of ruins. But in many cases, basaltic craters have ejected lava of scoriaceous texture, cinders, and scoriæ, like those of actual volcanoes. The Basalts, like the Trachytes, cover with a dome-shaped mass, the passage by which they have been ejected. Generally the Basalts with their tufas and conglomerates are of the tertiary period. They traverse all the stratified rocks and all eruptive rocks anterior to this age. The Basalts of central France appear to be later than the Trachytes of the same region. They generally appear in the form of prismatic columns, often rising vertically to considerable heights and are sub-divided transversely by joints, as at Fingal's Cave and Giant's Causeway. Zirkel undertook the microscopic study of Basaltic rocks in 1870. He discerned in a large number, a vitreous substance of a brownish-yellow color, sometimes gray or colorless. This matter plays the part of a cement to the crystalline grains. It represents the paste of the Basalts, the residue of the original magma, remaining in the amorphous state after the separation of the crystals. In some Basalts this paste is only demi-vitreous made up in part of black or brownish-black crystalline needles, to which Zirkel has given the name Trichites. Augite, often in crystals of a bright black color, plainly visible to the naked eye, becoming brown, more or less yellowish in thin sections, and exhibits colors in polarized light. It ordinarily offers the following forms (Fig. 22). It often contains much smaller crystals of Apatite and Magnetic Iron; also cavities empty or filled with liquid; perhaps compressed carbonic acid. Similar cavities are found in grains of Peridot. Feldspar is crystallized in elongated lammæ which are subdivided into longitudinal bands, alternately colored blue and gray, or with lively shades varying in polarized light. Zirkel observes that this Feldspar is not attacked by acids. Magnetic Iron is found in the form of irregular grains or crystals arranged in regular rows, disposed in files, or in interlacing, irregular rows, more or less regular. A great number of Basalts contain Nepheline and Amphigene. Accessory Elements: Apatite in elongated, hexagonal prisms; Hornblende, black or brown, traversed by parallel fissures; Magnesia; Mica, visible under the microscope. Accidental minerals: a great number of Zeolites (Chabasite, Analcime, Harmotome, Stilbite, etc.). Calc-spar, Aragonite; nodules of Hyalin Quartz, and Agates; small irregular masses of Hyalite in the cavities; Blue Sapphires, Cordierite; Black Pleonaste; Hyacinth Zircon, Pyrites, etc., mixed with the mass. Finally the Basalts of the county of Antrim, Ireland, contain sulphate of copper in solution, and owe this property, without doubt, to metallic Iron intimately mixed. Varieties; common Basalt. B. with fragments of Peridot. Peridotite of Cordier. The rock contains an aggregate of polyhedric or rounded grains of a green or yellowish-green or blackish-colored Peridot mixed with Enstatite. B. Amygdaloidal has large cavities in which may be found |