 It is known near Guatemala city as the Sierra de las Nubes, and enters Mexico as the Sierra de Istatan.Its summit is not a well-defined crest, but is often rounded or flattened into a table-land.The direction of the great volcanic cones, which rise in an irregular line above it, is not identical with the main axis of the Sierra itself, except near the Mexican frontier, but has a more southerly trend, especially towards El Salvador.