Popocatepetl (Aztec, " the smoking mountain "), a volcano about 45 m. S. S E. of the city of Mexico, in lat. 19° N, Ion. 98° 30' W., the largest of the six craters which succeed each other across the republic on the same parallel. It is an irregular cone of porphyritic obsidian, with an elevation, according to the latest measurements, of 17,540 ft. above the sea, or about 1,800 ft. higher than Mont Blanc. To the height of nearly 13,000 ft. it is covered with dense forests, those in the upper regions being almost exclusively of pine; for the last 3,000 ft. the sides lie beneath alternate layers of pumice and ashes, shrouded in eternal snow. The crater is about 3 m. in circumference, and has a depth of over 1,000 ft. Large quantities of sulphur are extracted from it, for which purpose several persons permanently reside within it, ascending and descending a large part of the way by ropes. The mountain is connected with Iztaccihuatl by a ridge, which at the pass of Ahualco is about 10,000 ft. above the sea. In 1519 Popocatepetl was in a state of extraordinary activity, and Cortes sent ten men under the leadership of Diego de Ordaz to climb to its summit. In 1522 Francisco Montafio reached the top, and was let down by ropes into the crater to a depth of about 450 ft.

In 1827 the brothers Frederick and William Glen-nie ascended to the highest point, and calculated the elevation of the volcano barometrically. More accurate calculations have since been made, particularly the last, by Sr. M. Ponce de Leon, in 1870.