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Updated: June 24, 2025
At Jedburgh the schistus was vertical, and the strata horizontal; and there was interposed a compound bed of pudding-stone, formed of various water-worn bodies, the gravel of the schistus strata, and porphyries. Here again, though we have not a regular pudding-stone, we have that which corresponds to it, as having been the effect of similar circumstances.
David's, consisting of diabasic and felsitic lava, with beds of ash; and in the centre of England, amongst the grits and slates of Charnwood Forest presumably of Cambrian age, various felstones, porphyries, and volcanic breccias are found.
The strata have in several places been tilted at small angles, generally either to N.N.W. or S.S.E. By gradual passages and alternations, the porphyries change incessantly in nature. I will describe only some of the principal mineralogical changes, which are highly instructive, and which I carefully examined.
Gillies, can be traced with little interruption for 140 miles southward to the R. Diamante, where it unites with the western ranges: northward, according to this same author, it terminates where the R. Mendoza debouches from the mountains; but a little further north in the eastern part of the Cumbre section, there are, as we shall hereafter see, some mountain-masses of a brick-red porphyry, the last injected amidst many other porphyries, and having so close an analogy with the coarse red granite of the Portillo line, that I am tempted to believe that they belong to the same axis of injection; if so, the Portillo line is at least 200 miles in length.
The very streets in the newly-formed parts of the city are macadamised with the fragments of costly baths and pillars. I took up one day, out of curiosity, some of the road-metal near the Church of Santa Maria Maggiore, and I identified in the handful no less than a dozen varieties of the most beautiful marbles and porphyries from Greece, Africa, and Asia.
The floor of most of them was formed in the style of mosaic known as "opus Alexandrinum," and the large sweeping, curved bands of coloured material with which the main outlines of the patterns are defined, and the general harmony of colour among the porphyries and other hard stones with which these pavements were executed, combine to satisfy the eye.
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.
They vary much in composition, resembling in many respects the dikes at Low's Harbour: the greater number consist of feldspathic porphyries, sometimes containing grains of quartz: one, however, was black and brilliant, like an augitic rock, but really formed of feldspar; others of a feldspathic nature were perfectly white, with either an earthy or crystalline fracture, and including grains and regular octagons of quartz; these white varieties passed into ordinary greenstones.
The manner in which the various kinds of porphyries and andesites have been injected one into the other, and in which the infinitely numerous dikes of various composition intersect each other, plainly show that the stratified crust has been stretched and yielded many times over the same points.
This brick-red porphyry appears to have burst through all the other porphyries, and numerous red dikes traversing the neighbouring mountains have proceeded from it: in some few places, however, it was intersected by white dikes. Having been accustomed to observe the irregularities of beds accumulated under currents, I feel sure that the inclination here has not been thus produced.
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