or nests is attacked; they then come stalking up, and attack the enemy with their strong jaws. Sometimes, when digging into the burrows, one of these giants has unperceived climbed up my dress, and the first intimation of his presence has been the burying of his jaws in my neck, from which he would not fail to draw the blood. The stately observant way in which they stalk about, and their great size, compared with the others, always impressed me with the idea that in their bulky heads lay the brains that directed the community in its various duties. Many of their actions, such as that I have mentioned of two relays of workmen carrying out the ant-food, can scarcely be blind instinct. Some of the ants make mistakes, and carry in unsuitable leaves. Thus grass is nearly always rejected by them, yet I have seen some ants, perhaps young ones, carrying in leaves of grass. After a while these pieces were invariably brought out again and thrown away. I can imagine a young ant getting a severe earwigging from one of the major-domos for its stupidity. I shall conclude this long account of the leaf-cutting ants with an instance of their reasoning powers. A nest was made near one of our tramways, and to get to the trees the ants had to cross the rails, over which the waggons were continually passing and repassing. Every time they came along a number of ants were crushed to death. They persevered in crossing for several days, but at last set to work and tunnelled underneath each rail. One day, when the waggons were not running, I stopped up the tunnels with stones; but although great numbers carrying leaves were thus cut off from the nest, they would not cross the rails, but set to work making fresh tunnels underneath them. Apparently an order had gone forth, or a general understanding been come to, that the rails were not to be crossed. These ants do not appear to have many enemies, though I sometimes found holes burrowed into their nests, probably by the small armadillo. I once saw a minute parasitic fly hovering over a column of ants, near a nest, and every now and then darting down and attaching an egg to one entering. Large, horned beetles (Coelosis biloba) and a species of Staphylinus are found in the nests, but probably their larvae live on the rotten leaves, after the ants have done with them. CHAPTER 6. Configuration of the ground at Santo Domingo. Excavation of valleys. Geology of the district. Decomposition of the rocks. Gold-mining. Auriferous quartz veins. Mode of occurrence of the gold. Lodes richer next the surface than at lower depths. Excavation and reduction of the ore. Extraction of the gold. "Mantos". Origin of mineral veins: their connection with intrusions of Plutonic rocks.