On the way to Buenos Ayres, the rugged Sierra de la Ventana, a white quartz mountain, was ascended. Buenos Ayres was reached on September 20, 1833, and no time was lost in arranging for an expedition to Santa Fé, nearly 300 miles up the Parana. On October 3, Santa Fé was entered, and near it many more remains of large extinct mammals were found. The remains of a horse, in a similar fossil condition, greatly astonished our explorer, for it seemed indeed surprising that in South America a native horse should have co-existed with giant extinct forms, and should itself have become extinct, to be succeeded in modern times by the countless herds descended from the few horses introduced by the Spanish colonists. These and other strange facts in the distribution of mammalian animals in America led Darwin to make some pregnant comments. The enormous number of large bones embedded in the estuary deposits became continually more evident, until he came to the conclusion that the whole area of the Pampas was one wide sepulchre. Unfortunately ill-health compelled the explorer to return, and on October 12th he started for Buenos Ayres in a small vessel. During this journey he had an opportunity of examining the shifting and variable islands of the muddy Parana, on which the jaguar thrives. Arrived at Las Conchas, a revolution had broken out, and Darwin was detained to a certain extent under surveillance; but by the influence of General Rosas' name, he was allowed to pass the sentinels, leaving his guide and horses behind, and ultimately reached Buenos Ayres in safety. After a fortnight's delay, Monte Video was once more made for. Here it appeared that the _Beagle_ would remain sometime longer, so the restless inquirer started on another expedition, this time up the Uruguay and Rio Negro. One of the halts was at the house of a very large landed proprietor. A friend of the proprietor's, a runaway captain from Buenos Ayres, was very anxious to have the traveller's opinion on the beauty of the Buenos Ayres ladies, and on receiving satisfactory assurances, voluntarily gave up his bed to the stranger! During this journey amazing quantities of huge thistles were met with, the cardoon being as high as a horse's back, while the Pampas thistle rose above the rider's head. To leave the road for a yard was out of the question. Incidentally the writer describes fully the horsemanship of the Gauchos, and gives a vivid picture of the state of society in the towns.