Chapter XV

Darwin observes that mules possess remarkable reason, memory, obstinacy, social affection, and endurance, suggesting that art has outdone nature in creating this hybrid animal.

Darwin notes that terrace formations of stratified shingle along Andean valleys resemble sediment deposited by torrents, leading him to infer gradual elevation of the Cordillera rather than sudden upheaval.

Darwin meditates on the enormous cumulative effect of river erosion over time, using the audible sound of stones as evidence that continents can be worn away by persistent geological processes.

Darwin explains that metallic veins project above ground due to hardness, and workers searching for firewood or herding cattle frequently stumble upon exposed ore deposits.

Darwin notes that rocks covered by snow for most of the year shatter into angular fragments, though he finds the mechanism obscure and proposes tentative explanations.

Darwin details the different compositions and formations of the Peuquenes and Portillo ridges, arguing that repeated upheavals and injections of different ages produced their parallel structure.

Darwin argues that fossil shells in the Cordillera prove the area must have subsided thousands of feet before being upraised, demonstrating that crustal elevation and subsidence are alternating processes.

Darwin explains that rivers drain through the Portillo ridge despite it being higher than the Peuquenes by invoking gradual elevation, which allowed channels to be eroded as the land rose.

Darwin reports experiencing slight respiratory difficulty (puna) at high altitude, discusses local misconceptions about its causes, and notes that fossil discoveries distracted him from discomfort.

Darwin observes red snow on high peaks, initially mistakes it for dust, then microscopically identifies it as colonies of spherical algae cells.

Darwin explains that the Cordillera intercepts the trade wind, drawing it northward and causing the southerly wind observed at high elevation to be an upper return current.

Darwin attributes the clear, brilliant atmosphere at high altitude to extreme dryness, evidenced by shrinking wood, hardened food, and unusual electrical phenomena.

Darwin argues that transparent air confounds distant objects by making them appear nearly coplanar, combined with unusual fatigue, explains why mountaineers misjudge distances.

Darwin demonstrates that water boils at lower temperature due to reduced atmospheric pressure, preventing potatoes from cooking despite prolonged boiling.

Darwin observes that plants and animals differ markedly between the eastern and western slopes despite similar climates, attributing this to the Cordillera as a historical barrier to migration.

Darwin notes that eastern Andean valleys contain characteristic Patagonian animals and plants, supporting the inference that the high plateau is geologically continuous with Patagonian plains.

Darwin climbs to view the pampas but finds them less spectacular than expected, resembling a distant ocean except for glittering rivers in the north.

Darwin describes an enormous swarm of locusts flying northward, comparing their sound to chariots and noting their devastating effect on vegetation once landed.

Darwin provides detailed observations of the benchuca (Reduvius) bug, describing its flat appearance before feeding and globular form after, and its aggressive feeding behavior.

Darwin describes the Uspallata range as composed of submarine lava and volcanic sediments resembling tertiary Pacific formations, differing from the Portillo range's granitic origin.

Darwin finds silicified tree trunks protruding from rock at high elevation, with volcanic sandstone layered around them showing the trees grew on land later submerged and upraised.

Darwin reconstructs the dramatic geological history: dry land with trees was raised from submarine depths, submerged under the ocean with alternating lava and sediment deposition, then re-elevated.

Darwin reports that the danger of Cordillera passes has been greatly exaggerated and notes that cargo mules are more frequently lost than riders due to shifting loads.

Darwin explains that the Incas Bridge consists of stratified shingle cemented by hot spring deposits, not a grand arch, and notes it is unworthy of its grandiose name.

Darwin expresses admiration for the natural politeness of Chileans, using an anecdote of men respectfully greeting a visibly disfigured woman to contrast with European attitudes.

Darwin observes that Mendoza is less impressive than Santiago, its prosperity has declined, and its inhabitants embody the idle, indolent character of Gauchos.

Darwin describes the eight small tower-like casuchas on the Uspallata pass, built to shelter travelers during winter, though now in poor condition.