Notes
The high table land of Mexico which we have described, is said to owe its present form to the circumstance that an ancient system of valleys in a chain of granitic mountains, has been filled up to the height of many thousand feet with various volcanic products. Five active volcanos traverse Mexico from west to east,—Tuxtla, Orizaba, Popocatepetl, Jorullo, and Colima. Jorulla which is in the centre of the great platform is no less than one hundred and twenty miles from the nearest ocean, which is an important circumstance, showing that proximity to the sea is not a necessary condition although certainly a very general characteristic of the position of active volcanos. If the line which connects these five volcanic vents in Mexico be prolonged westerly, it cuts the volcanic group in the Pacific called the group of Revilla-Gigedo.—Lyell's Geology, American edition, vol. I, p. 294.