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feet above the sea. It is called the Nevado, because usually its summit is white with snow. This volcano is isolated, being surrounded at some distance by volcanoes which have formed by the accumulation of their ash and lava an almost enclosed basin. It is one of the few high volcanoes of the world that can be ascended with ease, since it is possible to make the journey to and into the crater on horseback in four or five hours. Because of the ease with which it may be climbed the ascent has been made by a number of persons, the first of whom was the great geographer and traveler Humboldt, who reached the crater in 1803.

General Description.—Volcano Toluca is underlaid by calcareous rocks of Cretaceous age. The great mass of the volcano is composed of many layers of ash of varying degrees of thickness which conform quite closely to the slope. These layers of ash were apparently formed partly by the ash which rained down during the eruptions and partly by that which was carried down by streamlets and to a considerable extent in sheets during heavy rains. The accompanying photograph shows the stratified character of the slope and also a stratum of fossil soil, which in several of the "barrancas" or dry ravines is seen to be of considerable thickness. From this evidence it is fair to conclude that the last eruptions were preceded by a long period of inactivity, during which a large quantity of organic material was mixed with the weathered ash. Toluca has not been in eruption within historic times and at present there are no signs of activity, even secondary effects, such as fumaroles of steam and sulphur dioxide, being absent.