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PORPHYRITIC STRUCTURE. Rocks whose ground mass has scattered through it large conspicuous crystals are said to be PORPHYRITIC, and it is especially among volcanic rocks that this structure occurs. The ground mass of porphyries either may be glassy or may consist in part of a felt of minute crystals; in either case it represents the consolidation of the rock after its outpouring upon the surface.
Its total thickness, together with the upper beds of the porphyritic conglomerate, I estimated at least at 8,000 feet; and only a small portion of the porphyritic conglomerate, which on the eastern flank of the fourth axis of elevation appeared to be from fifteen hundred to two thousand feet thick, is here included.
During the cretaceo-oolitic period this certainly appears to have been the case at the Puente del Inca, judging from the number of intercalated lava-streams in the lower 3,000 feet of strata; but generally, the volcanic orifices seem at this time to have existed as submarine solfataras, and were certainly quiescent compared with their state during the accumulation of the porphyritic conglomerate formation.
Like an irresistible, all-compelling wave, the troops swept out of the valley to hurl themselves against castle and fortress and to plant their colors in the heart of the capital city. Clothed at its base in a misty raiment of purple, the royal hill lifted above the valley an Olympian crest of porphyritic rock into the fathomless blue.
At first the origin of this thick, massive, long but narrow formation, appeared to me very anomalous: whence were derived, and how were dispersed the innumerable fragments, often of large size, sometimes angular and sometimes rounded, and almost invariably composed of porphyritic rocks?
I recollect one interesting spot we came to in riding through the pine-forest on the northern slope of the mountains, where the course of a torrent, now dry, ran along a mere narrow trench in the hard porphyritic rock, some ten or fifteen feet wide, until it had suddenly entered a bed of gravel, where it had hollowed out a vast ravine, four hundred feet wide and two hundred deep, the inlet of the water being, in proportion, as small as the pipe that serves to fill a cistern.
These brecciated porphyries could generally be distinguished at once from the metamorphosed, porphyritic breccia- conglomerates, by all the fragments being angular and being formed of the same variety, and by the absence of every trace of aqueous deposition.
This whole pile resembles, but not very closely, some of the less brecciated parts of the great porphyritic conglomerate formation of Chile; but it does not probably belong to the same age, as the porphyries here rest unconformably on the altered feldspathic clay-slate, whereas the porphyritic conglomerate formation alternates with and rests conformably on it.
Further up, a broad porphyritic dyke crosses our path, then more trees, and we come to the gentle slope of a kind of granitic sand which composes the open space leading to the pass between Granite Chief on the right, and a peculiar battlemented rock, locally known as Fort Sumpter, on the left.
The first little range is composed of a brecciated purple porphyritic claystone, with obscurely marked strata dipping at 70 degrees to the S.W.; the other ranges consist of a pale-coloured feldspathic porphyry, a purple claystone porphyry with grains of quartz, and a rock almost exclusively composed of brick-red crystals of feldspar.
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