Art. LXXIV.—Notes on the Microscopic Structure of certain Igneous Rocks submitted by the Director of the Geological Survey of New Zealand. *
[Read before the Wellington Philosophical Society, 8th August, 1874.]
No. 207b.—(Selwyn River.) Altered dolerite. The constituents are plagioclase, augite, magnetite, and pseudomorphs after olivine. The large brown
[Footnote] * These minerals were collected by Dr. Haast. The composition of several of them is given in the Seventh Annual Report on the Colonial Laboratory, 1872, p. 17.
patches appear to be cavities filled with the fine-grained basaltic portions of the rock.
No. IX.—(Snowy Peak Range.) Similar to No. 207b.
No. 410.—(Haurata District.) This rock differs from those classed under this head, as orthoclase and plagioclase are both present. The brown mineral is probably augite; the colours, however, as seen in polarized light are far less brilliant than usual.
The base contains an immense number of microlites, and exhibits well the “fluidal” structure. Under a 1/4 inch objective the microlites appear of a clear yellowish brown; they are probably augite.
No. 218.—(Flagstaff Hill Basin.) This specimen consists of a fine-grained matrix composed of small grains and crystals of augite, plagioclase, and magnetite, in which larger crystals of olivine are imbedded.
No. 398.—(Haurata District.) Contains the same minerals as No. 218, together with slender acicular crystals of apatite.
The olivine has been much decomposed, and hydrous ferric oxide in some cases only remains.
No. 204.—(Acheron Section.) A weathered specimen. The felspar is very much altered; some appears to be orthoclase. There are many long prisms of apatite, and the small hexagonal crystals are the transverse sections of the prisms.
No. 308.—(Mount Misery.) Contains crystals of quartz and orthoclase in a compact felsitic base.
No. 358.—(Snowy Peak Range.) A similar rock of greenish colour. In addition to the quartz and felspar it contains a few garnets.
No. 359.—(Snowy Peak Range.) This appears to be the same as No. 358 in a fragmental condition, produced probably by the crushing action of the mass when in motion at the time of eruption.
No. 366.—(Snowy Peak Range.) This rock contains crystals of quartz and orthoclase, a few grains of garnet, and a little brown mica. It exhibits the fluidal structure very well, and, as part of the base is a structureless glass, it may be regarded as intermediate between felsite and pitchstone.
No. 348.—(Snowy Peak Range.) Consists of orthoclase, a considerable quantity of plagioclase (probably oligoclase), and a little silvery mica.
There is a yellowish mineral with a fibrous radial structure seen both in the specimen and section. It is evidently a secondary formation, filling spaces between the constituents. It is probably prehnite.
No. 349.—(Snowy Peak Range.) Contains crystals and crystalline grains
of orthoclase and quartz. The matrix in which they are imbedded is a structureless glass, densely crowded with an immense number of very minute yellowish brown granules, nearly uniform in size, and quite translucent. Black grains of magnetite are also scattered through the mass. The section cut contains a pale red crystal of irregular form, which exhibits no double refraction—it is evidently garnet.
Some of the orthoclase contains numerous cavities filled with brown glass, and one of the quartz crystals contains a characteristic rhomboidal cavity with a vacuity and five or six belonites.
The “fluidal” structure, as it has been called, is remarkably well shown; streams of microlites and brown glass bend round the larger crystals, and clearly indicate the plastic condition of the mass subsequently to their formation.
No. 353 (Snowy Peak Range) is a rock of quite similar character.