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Hence the above fossils here lie at the very base of the gypseous or cretaceo-oolitic formation, and hence they were probably once covered up by strata about seven thousand feet in thickness: it is, however, possible, though from the nature of all the other sections in this district not probable, that the porphyritic claystone lava may in this case have invaded a higher level in the series.
From the Cataract of the Macquarie, a flat extends to the marshes in which that river exhausts itself. From the midst of this flat Mount Foster and Mount Harris rise, both of which are porphyritic: but as I have been particular in describing these heights in their proper place, any minute notice of them here may be considered unnecessary.
Decomposing into Spherical Masses. Rude columnar Structure. Graphic Granite. Mutual Penetration of Crystals of Quartz and Feldspar. Glass Cavities in Quartz of Granite. Porphyritic, talcose, and syenitic Granite. Schorlrock and Eurite. Syenite. Connection of the Granites and Syenites with the Volcanic Rocks. Analogy in Composition of Trachyte and Granite.
It consists essentially of silicon, oxygen, iron; or, to speak more correctly, it is a silicate of the protoxide of iron. It is, in fact, a true igneous rock. Portions of quartz and silica still remaining unfused, are often contained in the masses, which give to them, when broken, a true porphyritic appearance, while, from the great preponderance of the protoxide of iron, it is invariably black.
These red and green rocks are often quite distinct, and often pass into each other; the passage being sometimes affected by a fine brecciated structure, particles of the red and green matter being mingled together. Some of the varieties appear gradually to become porphyritic with feldspar; and all of them are easily fusible into pale or dark-coloured beads, strongly attracted by the magnet.
Then he remembered. It was toward the pillars of the First National Trust Building that his mind was trying to grope. They were of the same stuff, highly polished porphyritic granite, the pride and wonder of the avenue along which they made a burnished and flashing peristyle. Trotter rubbed his chin, meditatively, and once more examined the stone.
This ridge, and likewise the Barn and Flagstaff Hills, are interlaced by dikes, many of which preserve a remarkable parallelism in a N.N.W. and S.S.E. direction. The dikes chiefly consist of a rock, porphyritic with large crystals of augite; others are formed of a fine-grained and brown- coloured trap.
Our afternoon was hot and unpleasant: about later March the Hawa el'-Uwwah, a violent sand-raising norther, sets in and lasts through a fortnight. After that the summer Jehannum! From the raised and metalled bank, upon which the Burj stands, we descended to the broad mouth of the Wijh valley, draining the low rolling blue-brown line of porphyritic hillocks on the east.
In descending from the Arqueros district, I crossed on the northern border of the valley, strata inclined eastward from the Pluclaro axis: on the porphyritic conglomerate there rested a mass, some hundred feet thick, of brown argillaceous limestone, in parts crystalline, and in parts almost composed of Hippurites Chilensis, d'Orbigny; above this came a black calcareous shale, and on it a red conglomerate.
This pile of strata differs considerably from the basal strata of the Cordillera in Central Chile, and may possibly belong to the upper and gypseous series: I saw, however, in the bed of the valley, one fragment of porphyritic breccia-conglomerate, exactly like those great masses met with in the more southern parts of Chile.
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