The vegetation gave evident proof of a severe climate; and yet the heat and glare of the sun were more intolerable than we had ever felt it in the region of sugar-canes and bananas.

About here, some of the trachytic porphyry which forms the substance of the hills had happened to have cooled, under suitable conditions, from the molten state into a sort of slag or volcanic glass, which is the obsidian in question; and, in places, this vitreous lava--from one layer having flowed over another which was already cool--was regularly stratified.
The mines were mere wells, not very deep; with horizontal workings into the obsidian where it was very good and in thick layers.

Round about were heaps of fragments, hundreds of tons of them; and it was clear, from the shape of these, that some of the manufacturing was done on the spot.

There had been great numbers of pits worked; and it was from these "minillas," little mines, as they are called, that we first got an idea how important an element this obsidian was in the old Aztec civilization.

In excursions made since, we travelled over whole districts in the plains, where fragments of these arrows and knives were to be found, literally at every step, mixed with morsels of pottery, and here and there a little clay idol.