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On the left bank a porphyritic block, up whose side a mule can be ridden, is disposed in a slope of the palest and most languid of greens, broken by piles of black rock so regular as to appear artificial. This step leads to a horizontal crest, a broken wall forming its summit: it is evidently an outlier; and experience asked, What will be behind it?
Near Paipote, also, there was much true porphyritic breccia-conglomerate: nevertheless, few of these masses were metamorphosed to the same degree with the corresponding formation in Central Chile.
I have only met with one account of an experiment made in a horizontal direction, and it is curious that the law of the increase of temperature then observed seemed to be very much the same as that determined by the mean of the vertical observations. Boussingault found several horizontal adits in a precipitous face of porphyritic syenite among the mountains of Marmato.
The strata are inclined, as before stated, from 20 to 30 degrees eastward, towards an irregular north and south chain of andesitic porphyry and of porphyritic greenstone, where they are abruptly cut off.
In Central Chile, from the extreme metamorphic action, it is in most parts difficult to distinguish between the streams of porphyritic lava and the porphyritic breccia-conglomerate, but here, at Copiapo, they are generally perfectly distinct, and in the Despoblado, I saw for the first time, two great strata of purple claystone porphyry, after having been for a considerable space closely united together, one above the other, become separated by a mass of fragmentary matter, and then both thin out; the lower one more rapidly than the upper and greater stream.
The limestone, however, with these fossils here lies at the very base of the formation, just above the porphyritic conglomerate, and certainly several thousand feet lower in the series, than the equivalent, fossiliferous, black, shaly rocks high up on the Peuquenes range.
Without supposing a movement of this kind, it would, moreover, be impossible to understand the accumulation of the several lower strata of COARSE, well-rounded conglomerates, which it is scarcely possible to believe were spread out in profoundly deep water, and which, especially those containing pebbles of quartz, could hardly have been rounded in submarine craters and afterwards ejected from them, as I believe to have been the case with much of the porphyritic conglomerate formation.
These veins, two or three inches thick, are distinguished by a fine-grained quartz-granite crossing a coarse-grained granite almost porphyritic, and abounding in fine crystals of red feldspar. I sought in vain, in the Cordillera of Baraguan, for hornblende, and those steatitic masses that characterise several granites of the Higher Alps in Switzerland.
The uppermost beds of the porphyritic conglomerate, on which the gypseous strata conformably rest, are variously coloured, with one very singular and beautiful stratum composed of purple pebbles of various kinds of porphyry, embedded in white calcareous spar, including cavities lined with bright-green crystallised epidote.
Ahmed Kaptan fixed them, by solar observations, in north lat. 26 13', so that we made only one mile of southing. It ignores the porphyritic sub-range in which the "Mother of the Villages" lies: and it brings close to the east of it the tall peaks of the Tihamat-Balawiyyah' which, from this point, rise like azure shadows on the horizon. Finally, it corrupts Umm el-Karayat to Feyrabat.
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