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Updated: June 8, 2025
It has already been observed, that from the level of the sea to Portillo, and as far as the entrance on the elevated plain of the Retama, that is, two-thirds of the total height of the volcano, the ground is so covered with plants, that it is difficult to make geological observations.
From this feldspar being orthitic, and even from its external appearance, I venture positively to affirm that it has not been derived from the rocks of the western ranges; but, on the other hand, it may well have come, together with the quartz and metamorphic schists, from the eastern or Portillo line, for this line mainly consists of coarse orthitic granite.
A little beyond the Portillo, the road diverges from the valley proper of the river, and is carried over an undulating country to the village of San Antonio del Norte, finely situated on a grassy plain, of considerable extent, a dependency of the valley of the Goascoran.
The commandant-general Aristizabal then made a similar communication to the archbishop of Cuba, Don Fernando Portillo y Torres, whose metropolis was then the city of St. Domingo, hoping to receive his countenance and aid in this pious undertaking.
Evidence derived from an inclined stream of lava at the eastern base of the Portillo, might be adduced to show, that it owes part of its great height to elevations of a still later date. Looking to its earliest origin, the red granite seems to have been injected on an ancient pre-existing line of white granite and mica-slate.
In the evening the stream is muddy and full, but about daybreak it becomes clearer and much less impetuous. This we found to be the case with the Rio Vacas, and in the morning we crossed it with little difficulty. The scenery thus far was very uninteresting, compared with that of the Portillo pass.
Whether the axis of the Uspallata range was injected before, or as perhaps is more probable, after that of the Portillo line, I will not pretend to decide; but it is well to remember that the highly inclined lava-streams on the eastern flank of the Portillo line, prove that its angular upheavement was not a single and sudden event; and therefore that the anticlinal elevation of the Uspallata range may have been contemporaneous with some of the later angular movements by which the gigantic Portillo range gained its present height above the adjoining plain.
We had seen its white church from El Portillo, looking like a point of silver on the dark green slope of the mountain. Rain fell heavily during the night; but the morning broke bright and clear.
Hence the potatoes, after remaining for some hours in the boiling water, were nearly as hard as ever. The pot was left on the fire all night, and next morning it was boiled again, but yet the potatoes were not cooked. After eating our potato-less breakfast, we travelled across the intermediate tract to the foot of the Portillo range.
I was astonished to find that this conglomerate was partly composed of pebbles, derived from the rocks, with their fossil shells, of the Peuquenes range; and partly of red potash-granite, like that of the Portillo. So that the Portillo, the loftiest line in this part of the Cordillera, is not so old as the less lofty line of the Peuquenes.
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