From the Rio de las Vacas to the Puente del Inca, half a days journey; here was a little pasture for the mules; & some interesting geology for me, so we bivouaced for the night. When one hears of a Natural bridge, one pictures to oneself some deep & narrow ravine across which a bold mass of rock has fallen, or a great archway excavated. Instead of all this the Incas bridge is a miserable object. The bottom of the valley is nearly even & composed of a mass of Alluvium; on one side are several hot mineral springs, & these have deposited over the pebbles a considerable thickness of hard stratified Tufa; The river running in a narrow channel, scooped out an archway beneath the hard Tufa; soil & stones falling down from the opposite side at last met the over hanging part & formed the bridge. The oblique junction of the stratified rock & a confused mass is very distinct & this latter is different from the general character of the plain. This Inca's bridge is truly a sight not worth seeing.
Near to this place are some ruins of Indian buildings; they consist now merely of the vestiges of walls; I saw such in several other stations; the most perfect were the Ruinas del Tambillos. — The rooms were small & square & many huddled together in distinct groups; some of the doorways yet stood, these were formed of a cross slab of stone & very low, not more than 3 ft high. — The whole were capable of containing a good many people. Tradition says they were the halting places for the Incas when they crossed the Cordilleras, & these Monarchs would probably travel with a large Retinue. The situation of Tambillos is utterly desert & that of the Puente only a shade better. Traces of Indian buildings are common all over the Cordilleras; those mentioned in the Portillo pass probably were not only used as lodging houses in the passage; because if so, there would have been others, & the situation is by no means central. — Yet the Valley is now quite useless & destitute of vegetation. — In the ravine of Jajuel near Aconcagua I frequently heard of numerous remains situated at a great elevation, & of course both cold & sterile; — there is no pass in that part. —I at this time imagined these might have been places of refuge on the first arrival of the Spaniards. Subsequently what I have seen has led me almost to suspect there has been a change of Climate in these Latitudes.2 In very many places, indeed in all the ravines, in the Cordilleras of Copiapo remains of Indian houses are found; in these they find bits of woollen articles, instruments of precious metals, Indian corn, & I had in my possession the head of an arrow made of Agate, of precisely the same figure as those in T. del Fuego. — It is the opinion of the people of the country that the Indians resided in these houses; Now I am assured by men who have passed their lives in travelling the Andes, that these ruins are found at the greatest elevations, almost on the limit of perpetual snow, in places where there are no passes, where the ground produces nothing, & what is more extraordinary where there is no water. In the "Despoblado" (ininhabited) valley near Copiapò at a spot called Punta Gorda, I saw the remains of seven or eight square little rooms; they were of a similar form with those at the Tambillos but chiefly built of mud instead of stone, & which mud the people of the country cannot imitate in hardness: there was no water nearer than 3 or four leagues & this only in small quantity & bad. — The valley is utterly desert.— These houses are placed in the most conspicuous spot in a broad flat valley & in a defenceless position; they could not therefore have been places of refuge. — Even with the advantage of beasts of burden, a mine could only be worked here at great expense; yet former Indians chose it out as a place of residence. A person who has never seen such countries will not readily understand how entirely unfit they are for human habitations. If however a few showers were to fall annually, in the place of one in several years, so as to make a small rill of water, by irrigation such spots would be highly fertile. — All these facts strongly incline me to suspect that some change for the worse has taken place since the period when the ruins were inhabited. — (Note in margin: The Indians in the Quebrada of [illegible] had built an extensive Azequi or Conduit with the hard mud.)
I have certain proof that the S. part of continent of S. America has been elevated from 4 to 500 feet within the epoch of the existence of such shells as are now found on the coasts. It may possibly have been much more on the sea-coast & probably more in the Cordilleras. If the Andes were lowered till they formed (perhaps 3–4000 ft) a mere peninsula with outlying Islands, would not the climate probably be more like that of the S. Sea Islands, than its present parched nature — At a remote Geological æra, I can show that this grand chain consisted of Volcanic Islands, covered with luxuriant forests, some of the trees one of which, 15 feet in circumference, I have seen silicified & imbedded in marine strata. — If the mountains rose slowly, the change of climate would also deteriorate slowly; I know of no reason for denying that a large part of this may have taken place since S. America was peopled. — We need not be surprised at the remains of stone & hardened mud walls lasting for so many ages as I imagine; it will be well to call to mind how many centuries the Druidical mounds have withstood even the climate of England. — I may also remark that the above conjecture explains the present elevation of the ruins; I am aware that the Peruvian Indians chose stations so lofty that a stranger is affected with Puna, but I am assured there are "muchissimas" houses where during the whole long winter snow lies. Surely no people would found a village under such circumstances. — When at Lima3 I was conversing with a civil engineer, Mr Gill, about the number of Indian ruins & quantity of ground thrown out of cultivation in that province, & he told me that the conjecture about a change of climate had sometimes crossed his mind; but generally he thought that the present sterility where there was formerly cultivation was chiefly owing to neglect or subterranean movements injuring the Conduits or subterranean passages, which the Indians had formed on so wonderful a scale to bring water for the purposes of irrigation. — As an illustration he told me one very curious fact, that travelling from Casma to Huaraz he found a plain covered with ruins &c &c & now quite bare; near to it was the dry course of a considerable river; in its bed there were pebbles & sand, & in one spot solid rock to the depth of 8 feet & about 40 yards wide had been cut through. (Note in margin: The fall in perpendicular ft. about 40–50.) From its appearance he could not tell that the river had not followed this line within a few years; but upon following up the course for a short distance, to his astonishment he afterwards found it going down hill; that is the bed of the river was arched; this could, of course, only happen after some subterranean movement which would throw the water back on itself untill some new lateral line of drainage was opened.4 The inhabited plain from that year would necessarily be deserted.