These porphyries are pierced by greenstone two or three feet thick, and the granites are intersected by numerous veins of amphibolite (hornblende) and greenstone, generally running from east to west.
Masses of euritic and porphyritic rocks intersect the granites, and a distinct formation of porphyries crowns Monte Cinto, Vagliorba, and Pertusato, the highest summits of the Niolo, covering the granite.
Among the other mineral products may also be mentioned the porphyries of the Limbara, the basalt of Nurri, Gestori, and Serri, the alabaster of Sarcidanu, and the marbles of the Goceano and Monte Raso.
In one part, there were many pebbles of the common metamorphic porphyries of the Cordillera: and near the base of the cliff, I observed a single rounded boulder of greenstone, nearly a yard in diameter.
This earthy substance often becomes coarser, and contains minute rounded fragments of porphyries and rounded grains of quartz, and in one case so many of the latter as to resemble a common sandstone.
One of the commonest stratified porphyries is of a green colour, highly amygdaloidal with the various minerals described in the preliminary discussion, and including fine tabular crystals of albite.
In these upper portions the porphyries are generally lighter coloured.
The pebbles are of claystone porphyries of many varieties; they are tolerably well rounded, and vary in size from a large apple to a man's head.
By gradual passages and alternations, the porphyries change incessantly in nature.
Geikie's view to be correct, it is possible that we may have in the granite and quartz-porphyries of Mourne and Carlingford representatives of the granites, granophyres, and other acid rocks of the later period of Mull.
Volcanic rocks of Devonian age occur in the South of Scotland, consisting of felstone-porphyries and melaphyres; also at Boyle, in Roscommon, and amongst the Glengariff beds near Killarney in Ireland.
The porphyries possess various degrees of hardness and compactness.
A few words will suffice to acquaint the reader with the metamorphism exercised by the porphyries and granites.
In this country, and even in France, where the climate is much drier, the porphyries frequently decompose so as to become scarcely recognisable.
They are thus distinguished from the porphyries and porphyrites in which the felspar has crystallized in two generations.
D'Orbigny considers the porphyriesof the Andes to have been ejected at the close of the cretaceous period, and formed the first relief of the Cordillera.
They differ from the Old World porphyries in containing no quartz, and seldom mica.
The normal dark-green traps and burnished red porphyries and grits were sparsely clad with the Shauhat and the Yasár trees, resembling the Salvadora and the Tamarix.
They attract attention only by their material, red boulders being used instead of the green porphyries of the hills; and the now desolate spot shows no signs of water or of palm-groves.
The porphyries containing vitreous feldspar and hornblende, the phonolite, the greenstone, the amygdaloids, and the basalt, have forms almost as invariable as simple crystallized substances.
They pass into pitch-stone; and we may consider them as porphyries with a basis of obsidian.
It appears to me at present extremely probable, that obsidians, and porphyries with bases of obsidian, are vitrified masses, the cooling of which has been too rapid to change them into lithoid lava.
The mountains of mica-slate and gneiss in Peru and New Grenada immediately touch the volcanic porphyries of the provinces of Quito and Pasto.
Sometimes both varieties meet in the same fragment, as we observed also in the trappean porphyries of the valley of Mexico.
Later views favor solutions coming directly from theporphyries or deeper igneous sources.
But some of these same porphyries are partially unconformable, as b, and may lead us to suspect that the others also, notwithstanding their appearance of interstratification, have been forcibly injected.
Professor Sedgwick supposes these porphyries to have originated contemporaneously with the stratified chloritic slates, the materials of the slates having been supplied, in part at least, by submarine eruptions oftentimes repeated.
Granite, syenite, and those porphyries which have a granitiform structure, in short all plutonic rocks, are frequently observed to contain metals, at or near their junction with stratified formations.
I believe that this rock belongs to the porphyries of Glendon, and of the upper Gloucester.
Against its nucleus of very ancient granites and porphyries the Ozark series of magnesian limestone was laid down.
The porphyries issue (like the trachytes of the Andes) in domes from the bosom of intermediary rocks.
These garnets re-appear in the trachytic porphyries that crown the celebrated metalliferous mountain of Potosi, and in the black and pyroxenic masses of the small volcano of Yana-Urca, at the back of Chimborazo.
Next to the pumice-tuffs, massive quartz-porphyries and oligoclase-trachytes are the characteristic rocks.
In connection with the quartz-porphyries and trachytic rocks which also occur in Vanua Levu, it should be observed that Mr. Andrews describes a rhyolite from Suva in Viti Levu.
Exposure of altered trachytes and quartz-porphyries at its base, 215.
They are derived from the degradation of quartz-porphyries or rhyolitic rocks.
Glassy forms of the quartz porphyries or intrusive rhyolitic rocks are extensively represented in the pumice tuffs of the Undu Promontory and of the coasts between the Langa-langa river and Lambasa.
Granite, syenite, and those porphyries which have a granitiform structure, in short all Plutonic rocks, are frequently observed to contain metals, at or near their junction with stratified formations.
But some of these same porphyries are partially unconformable, as b, and may lead us to suspect that the others also, notwithstanding their appearance of interstratification, have been forcibly injected.
The above list will hopefully give you a few useful examples demonstrating the appropriate usage of "porphyries" in a variety of sentences. We hope that you will now be able to make sentences using this word.
